Proceedings of the Ussher Society

Research into the geology and geomorphology of S. W. England Volume 5 Part 1 1980

Edited by R.A. Edwards

The Ussher Society

Objects: To promote research into the geology and geomorphology of South West England and the surrounding marine areas; to hold Annual Conferences at various places in South West England where those engaged in this research can meet formally to hear original contributions and progress reports and informally to effect personal contacts; to publish, proceedings of such Conferences or any other work which the Officers of the Society may deem suitable.

Officers: Chairman Dr C.S. Exley Vice-Chairman Mr C.M. Bristow Secretary Dr R.T. Taylor Treasurer Dr J.M. Thomas Editor Dr R.A. Edwards Committee Members Dr K. Atkinson

Mr P. Grainger Dr J.T. Renouf Prof J.W. Murray Dr LP. Tunbridge

Membership of the Ussher Society is open to all on written application to the Secretary and payment of the subscription due on January lst each year.

Back numbers may be purchased from the Secretary to whom correspondence should be directed at the following address:

Dr R.T. Taylor, Institute of Geological Sciences, St Just, 30 Pennsylvania Road, Exeter EX4 6BX Devon.

Proceedings of the

Ussher Society

Volume 5 Part 1 1980

Edited by R.A. Edwards

Crediton, 1980

© Ussher Society

ISSN 0566-3954

1980

Tipeset, printed and bound bv Phillips & Co., The Kyrtonia Press, 115 High Street, Crediton, Devon EXl73LG

Set in Monophoto Times and Printed by Photolithography

Proceedings of the Ussher Society Volume 5, Part 1, 1980

Chairman's Report 1

Papers

C. NICHOLAS: Developments in economic geology in south-west England - a review of the 1970s

2 I.P. TUNBRIDGE: The Yes Tor Member of the Hangman Sandstone Group (North Devon)

7

STEPHEN KERSHAW and ROBERT RIDING: Stromatoporoid morphotypes of the Middle Devonian Torbay Reef Complex at Long Quarry Point, Devon

13

G. WARRINGTON and R.C. SCRIVENER: The Lyme Regis (1901) Borehole succession and its relationship to the Triassic sequence of the east Devon coast

24

W.R. DEARMAN, B.E. LEVERIDGE, R.P. RATTEY and D.J. SANDERSON: Superposed folding at Rosemullion in south-west Cornwall

33

R.P. RATTEY: Deformation in south-west Cornwall 39 J.P.N BADHAM: Late magmatic phenomena in the Cornish batholith - useful field guides for tin mineralisation

44

R.C SCRIVENER and M.J. BENNETT: Ore genesis and controls of mineralization in the Upper Palaeozoic rocks of north Devon

54

G.M. POWER and W. GIBBONS: Field relations and geochemistry of the foliated granitic sheets of Sark, Channel Islands

59

P.A. FLOYD and A.H. AL-SAMMAN: Primary and secondary chemical variation exhibited by some west Cornish volcanic rocks

68

J. DANGERFIELD, J.R. HAWKES and E.C. HUNT: The distribution of lithium in the St Austell Granite

76

E.M. DURRANCE, R.E. MEADS, R.K. BRINDLEY and A.G.W. STARK: Radioactive disequilibrium in uranium-bearing nodules from the New Red Sandstone (Permian-Triassic) of Budleigh Salterton, Devon

81

Notes and Abstracts

M. EVANS: Note on a fossiliferous Meadfoot Group locality at Punch's Cross, Polruan, Cornwall

89 G. WARRINGTON: Palynological studies of Triassic rocks in central Somerset (Abstract)

90

G. WARRINGTON: British Triassic palaeontology: supplement 90 P.J. LOVELAND: Heavy minerals in the Upper Albian/ Lower Cenomanian 91 GILBERTSON and A.B'. HAWKINS: An early Flandrian sea-level in the Severn Estuary

92

B.B. CLARKE: Geomorphology of the Camel Valley and Estuary (Abstract) 93 ATKINSON and P.C. STETHRIDGE: A preliminary report on the use of terrestrial photogrammetry in the study of rock slopes in Cornwall

94

Conference of the Ussher Society held at Plymouth, January 1980 CHAIRMAN'S REPORT 1980 saw the Conference back again at the College of St Mark and St John on the northern outskirts of Plymouth. The Conference was well attended and, thankfully, the freak wintry conditions which caused the cancellation of the 1979 Conference excursions were absent. However, for the first time in its history the Ussher Society organised, in conjunction with the Royal Geological Society of Cornwall, a meeting in addition to the usual January Conference, at the Fourth Meeting of the Geological Societies of the British Isles, held in Sheffield in September 1979. This meeting was entitled "Recent Developments in Geological Research in the South-West of England" and was both organised and chaired by Dr K. Atkinson of the Camborne School of Mines, to whom we are extremely grateful. Four papers were presented: Dr D. Alderton gave a talk entitled "Post-magmatic events associated with the granite batholith"; Dr A.V. Bromley talked about "A summary of recent views on the geology of the Lizard Complex"; Dr E.C. Freshney on "The Tertiary of south-west England"; and Dr C. Nicholas talked about "Develop-ments in economic geology during the 1970s". The meeting was well received and generally considered to have been well worth the effort which went into its organisation. Dr Nicholas' paper is published in this part of the Proceedings. The Annual Conference in Plymouth had a full and varied Programme beginning with .an excellent lecture by Mr Douglas Hamilton, our Guest Speaker, entitled "Are the present-day shelf sediments of the South-Western Approaches to Britain a key to the past?". This was followed by a group of papers on Quaternary and geomorphological topics. Some micropalaeontological work on the Trias of Central Somerset and the Cenomanian of South Devon was then reported, and the sedimentology of the Yes Tor member of the Hangman Sandstones reviewed. Some structural papers then rounded off the day. On the second day two papers on mineralisation were presented, follow/ed by some applied geology in the form of terrestrial photogrammetry as it can be used for studying cliff stability, and an account of attempts to identify shallow old mine workings by geophysical methods. The afternoon was devoted to the Cadomian granitic rocks of Sark, the geochemistry of volcanic rocks from south Cornwall, and the distribution of lithium in the St Austell granite.

A field excursion on Saturday 5th January to the Kit Hill and Callington area was well attended and benefited from excellent weather. Our thanks to Dr C. Bowler for leading this field trip. The system of recording discussion was revised in 1980 so that those who asked questions were requested to write their questions down on a slip of paper, with the speaker then providing his answer subsequently to the Editor. It is hoped that this system will be more practical in operation and still give us a record of the discussions, which often have some highly meaningful contributions in them. It is worth noting that our finances have remained strong in recent years, with some £2,500 of reserves in the Ussher Fund. At the AGM it was felt that this money could well be spent on bringing an Invited Speaker from abroad for 1981, and it is hoped that this wilt materialize. Further reprinting of out-of-print back numbers of the Society's Proceedings is being undertaken to increase our ability to offer whole sets of back numbers. Also, a change in the format of the Proceedings was agreed at the AGM, to enable text diagrams to be more satisfactorily presented and to provide a more attractive and flexible layout for the journal. 1980 was therefore a year of change for the Society, and I would like to pay tribute to the Officers and Committee, who have put in so much work this year, and especially to Dr M.B. Hart who retires this year as Honorary Secretary, having served us so well for so many years. The Society is grateful to Richard Scrivener for compiling the index to Volume Four of the Proceedings. My best wishes to my successor, Dr Colin Exley, who, as one of the founding members of the Ussher Society, richly deserves the honour of being our Chairman. C.M. Bristow March 1980

Read at the Joint Meeting of Geological Societies of the British Isles, Sheffield, 20th September 1979. Session A 4: Recent developments in geological research in the south-west of England. Ussher Society and Royal Geological Society of Cornwall.

Developments in economic geology in south-west England - a review of the 1970s C. NICHOLAS E. C. C. Quarries Ltd., Northernhay House East, Northernhay Place, Exeter EX4 3QP Introduction The changes and developments which have occurred in applied geology in recent years are nationwide or even worldwide phenomena. The great increase in the involvement of geologists in widely varying spheres of "industry" (in the broadest sense of the word) is largely an effect of the 1970s. No less in south-west England. The numbers of geologists working in the area have increased during the decade from a few tens well into the hundreds. At the turn of the decade it is highly appropriate to look back at developments through the 1970s and perhaps in doing so to indulge in a little crystal ball gazing into the 1980s. In this review, south-west England is taken as the South West Economic Planning Region. 1970 saw the publication of the Regional Economic Map detailing a wealth of information on applied geological as well as other planning matters (Ministry of Housing and Local Government, 1970). This review, however, is somewhat biased towards the areas generally accepted as "Ussher Society Country" which is broadly south-west of a line from Bristol to Bournemouth. Geologists newly arrived in south-west England soon become aware that the general style of geology in this area is very different from that elsewhere in the country. This difference often comes as a surprise. First of all and most obvious to the outsider is that the south-west is properly in Hercynian Country with the resulting difference in stratigraphical and structural style. However, what does not show up on the geological map is the more subtle, yet much more far-reaching effect of the lack of glaciation with the absence of obscuring glacial sediments. Instead the widespread remnants of Tertiary tropical weathering combine with the results of peri-glacial effects, to give a variety of repercussions.

Engineering geology Early in the 1970s, the theme of a symposium at the annual meeting of the Ussher Society was "Some aspects of Engineering Geology in South West England" (Proceedings of the Ussher Society Vol. 2, Part 5, 1972). Nearly ten years on it seems that another Society symposium on the same theme is now due. The symposium was chaired by Fred Sherrell at a time shortly after the large scale road construction programme had just about started to push its way into the area. The new roads are more or less taken for granted now at the end of the decade and it is difficult to imagine what transport conditions Would have been like without them. However it seems that the different style of the geology is this area was equally surprising to the road-making teams, and it was certainly more forceful because their first real encounter with strange problems had been the Nags Head landslip in east Devon in the late 1960s. This sequence of landslips developed when an attempt was made to excavate a deep cutting for the Cullompton By-Pass. The remnant of this is seen on the M5 where a sign announces "hard shoulder narrows ahead". The landslips occurred in a major syncline in Carboniferous rocks immediately beneath the basal New Red Sandstone unconformity -- a geological horizon usually characterised by extensive weathering which was soon recognised as likely to give rise to geotechnical problems wherever it is encountered. The Nags Head problem was finally solved by the detailed work of Fred Sherrell and the application of hydrogeological principles to the design of a drainage scheme by means of an adit driven into and behind the slip area. All this has already been well documented (Sherrell 1971) but the point to be demonstrated is the significance of this study in all which has followed in the engineering geology of road developments in the south-west through the 1970s.

The change in attitude of the engineers is all important -gone now is the situation where geologists were brought in more or less as an afterthought to comment on a preconceived route and presented with a series of engineers' borehole logs drilled at regular intervals with two at a bridge. Experience has shown the value of the initial geological input in planning of the route from the preliminary (what has come to be known as) Engineering Geomorphological walk-over survey of all alternatives with the recognition of existing landforms, their origin and the assessment of their reaction when disturbed, through the planning and execution of site investigation and testing followed by the now inevitable public inquiry (The Okehampton By-Pass inquiry started in May 1979, and continued to the end of the year although the "public" seemed to lose much interest before the end), right through to the equally inevitable contractors' claims afterwards. The application of standard engineering geological and hydrogeological principles has enabled cuttings and embankments to be designed to best advantage with each case being considered on its geological merits. Quarrying The same pattern of evolution can be traced in quarrying. The nationwide trend for fewer but much larger quarries has been followed in the region and the large scale capital investment now involved in both mobile plant (excavators, dumpers, etc) and fixed plant (crushers, screens, etc) in production units of several hundred thousand tonnes per year and into the millions - units with more than 5 million tonnes per year capacity are now working in the Carboniferous Limestone of the Mendips - requires long term forward planning of reserves and working programmes. This, coupled with increased legislation and environmental requirements, has led to geologists again being fully involved throughout and indeed in many cases leading the planning programme. Deeper quarrying has necessitated studies of rock stability and hydrogeology for prediction of pumping requirements. As usual the problems have inevitably arrived first to be followed by geological studies and solutions applied back into prediction and forward planning. The effects of legislation such as the Mines and Quarries (Tips) Act (1969) and Regulations (1971) at the beginning of the decade has meant that tips which used to be just quarry and mine waste dumped over the side now have to be designed and built properly to detailed engineering geological principles and reported on at regular intervals. As well as this, some form of after use should be envisaged in the design but at the very least landscaping in keeping with the natural environment is usually a condition before planning permission will be granted.

China Clay Perhaps the major obvious effect of the combination of the Tips Regulations and the environmental requirements has been the change in appearance in the china clay districts. With the new regulations many of the old tips were studied and found to be unsatisfactory so that they often had to be dealt with immediately, sometimes by emergency construction of supporting beams. The old conical tips had to be reshaped and the design changed along with new tipping methods to give the longer, flatter conveyor tips which have appeared during the 1970s (Ripley 1972). The increases in scale have required detailed engineering geological design work for foundations and drainage especially since a decision was made to dispose of the fine slurry micaceous residues within the china clay working areas. These were previously allowed to run to local streams. Kernick Dam near St Austell is an example of one of these disposal areas where a high bund is being progressively raised across the neck of an abandoned, but not worked-out, clay pit using the sand wastes to impound the residue. This illustrates another aspect of geological work which is that it is very often necessary, usually for environmental reasons, to seek a balance between some sterilisation of reserves in order to permit waste materials from the remainder of the reserves to be dealt with. This requirement can also be illustrated by reference to the working of gravel from the Bunter Pebble Beds in east Devon where anything up to 30 per cent of the deposit may have to be sacrificed in the form of bunds between the working areas to permit silt washings to be back-filled into the old pits which are then capped with overburden and topsoil arising from elsewhere, grassed and returned to agriculture. Because it is the reserves/waste balance which is critical in all such cases the geological input to the working and restoration design is of vital importance and the evaluation of waste quantities and qualities becomes as necessary as the evaluation of the required mineral itself. Returning to the subject of china clay, the most desirable situation from the mineral reserves point of view is for the waste to be discarded in areas of barren ground and of course it goes without saying that the delineation of reserves areas is all important to avoid the old problem of the best clay lying beneath the biggest tip. The drill remains the most useful investigation tool, but perhaps the most valuable advances in the 1970s have been the developments in geophysical techniques whereby the already established resistivity method can now be supplemented by gravity surveying, and this has really been made possible in practice by the decade's giant strides in instrumentation and in electronics. The combination of resistivity and gravity surveying is now a very powerful tool to guide the eventual drilling programme.

Ball clay Turning now to ball clay, the influence of geological work has been no less felt here and perhaps the biggest advance here again has been in appreciation of the geological problems involved. The combined efforts of IGS (largely by the work of Ted Freshney) with the operating companies' geologists has resulted in awareness of the complexities of the deposits resulting from their origin add the appreciation of the sedimentology of the fluviatile/swamp environment of deposition (Freshney 1970). The necessity for closely controlled drilling with full core recovery and detailed inch-by-inch geological logging, sampling and chemical analyses has had to be coupled with improvements in drilling techniques especially wire-line coring (Vincent 1972). A recent development is the appreciation of the value of geophysical borehole logging techniques. This new awareness of the geological Complexities of the deposits which started to become apparent in the late 1960s and developed through the 1970s, has led to the successful reappraisal of-old working areas thought to have been exhausted, and of other areas previously explored and abandoned. An example of this was the successful exploration of the Arne Peninsula next to Poole Harbour, Dorset which had to be the subject of very lengthy planning procedures, including a public inquiry, before permission for working was given in 1977. Metal mining Staying with exploration we move to the most commonly known aspect of economic geology in the region, the metallic minerals, tin, tungsten, copper etc. The successful operations at Geevor Mine and at South Crofty Mine are well known and steady exploration and development continues around both properties with perhaps the most exciting being the seaward extension of Geevor by means of an incline to reach the lower levels of the old Levant Mine workings. Prospecting continues at numerous locations in west Cornwall by companies such as Consolidated Goldfields, Billiton, Central Mining Finance and St Piran. The Wheal Jane Mine is to be re-opened by Rio Tinto Zinc. The decision by Cornwall Tin and Mining Limited to abandon the nearby Mount Wellington Mine precipitated the Wheal Jane closure because of interconnecting water problems. Both these mines opened and closed in the 1970s. Fortunately the government provided finance to continue pumping because flooding would probably have meant complete abandonment, It is very important to note that at the time of closure the price of tin was about £6000 per tonne but within months the price had gone above £7000. There may be some slight cause and effect here because metal merchants are usually sensitive to mine closures but the result was that the economics of the operation immediately looked better. This illustrates what is happening all the time in this industry which is the rapid fluctuation in activity depending on price .

Elsewhere in the South West some exploratory work is being carried out in the Callington/Gunnislake area where South West Consolidated Minerals and Cominco are both exploring, the latter by means of a series of deep boreholes recently granted planning permission (1979) for drilling to investigate the ground beneath extensive old workings. Last but not least and perhaps the most exciting of all is the joint venture between Amax Exploration of UK Inc. and Hemerdon Mining and Smelting Limited at Hemerdon on the edge of Dartmoor immediately north-east of Plymouth. A six million dollar exploration programme involving 19000 metres of drilling has proved 45 million tonnes of extractable mineralised ground grading 0.17% WO3 and 0.025% Sn in a 650 metres long by 150 metres wide by 200 metres deep reserves block. Extraction by open pit would, in addition, require excavation of very substantial quantities of waste materials from the sides of the deposit to achieve the depth so that in all something in excess of 100 million tonnes of excavation and. disposal might be anticipated which brings us back to quite formidable problems of engineering geology and the environment. The Hemerdon prospect has now entered the feasibility study stage in which an Underground incline dipping at 1 in 4 along the length of the deposit is being sunk to provide sampling for pilot plant processing trials. Total expenditure of around 10 million dollars is anticipated before evaluation of the deposit is complete by late 1981. If the Hemerdon project comes to a successful conclusion, the mine will be one of the biggest tungsten operations in the world, albeit with a much lower grade of ore than many of the others. Other minerals In exploration for "other minerals" in the region perhaps brief mention needs to be made of intermittent small-scale interest in barytes in the Teign Valley and on Exmoor with the increase m requirements in the UK for oil drilling muds, but enthusiasm has usually declined as rapidly as it has arisen with the interested companies' appreciation of the environmental problems of mining in the National Parks. Some small but quite valuable production of fluorspar from old ups has continued through the 1970s and there is some interest in new areas as the established ones become exhausted. Some mention needs to be made of developments in "adding value" for specialist uses to some of the low price constructional materials. High-grade burnt lime for the South Wales steelworks has been brought into production from Batts Coombe Quarry in Mendip. Bagshot Sands in Dorset have been upgraded for industrial purposes principally as moulding sand to supply foundries in the south-west, saving transport costs from traditional suppliers in Surrey and in the Midlands.

Around 120000 tonnes per year of peat is produced from the Somerset levels, almost all for agricultural purposes. Exploration will probably need to be looked at more carefully as reserves with old planning permissions become worked out. Peat is one of the few minerals which can be subject to the hard-sell marketing approach; an advertising programme to sell a new variety of potting compost with a colourful flowery polythene package can double the sales overnight. No amount of advertising will sell a ton more sand for concrete or a ton more barytes for oil drilling - either there is a requirement or there isn't and this is a very important point to appreciate in mineral economics. Waste disposal Following on from the creation of holes by mineral extraction can be the very valuable use of the hole so created, by infilling With waste materials from domestic and industrial sources. Developments in the 1970s illustrate yet another instance of the effect of legislation and environmental require-ments. The Deposit of Poisonous Waste Act (1972) was followed closely by the Control of Pollution Act (1974) requiring the licensing and control of all waste disposal sites, with the County Councils as the licensing authorities. Geologically speaking, the main requirement of the legislation has been for hydrogeological studies to ensure that any waste tipping does not give rise to any unacceptable risk of water pollution. Old sites have had to be subject to detailed examination with the requirement for geological investigations to ensure that the leachates inevitably generated can be dealt with either by dilution and dispersion into the ground in permeable situations or by containment in impermeable situations where some form of treatment is usually required. Drilling programmes for water monitoring have had to be carried out at many sites in the West Country and both drillers and geologists have had to learn about the geotechniques of drilling through and predicting the physical and chemical behaviour of mixtures of rusty bikes and motor car tyres, polythene bags and empty tin cans. This is not a pleasant job but one in which many mineral extraction geologists have found themselves involved during recent years. Water Reorganisation of the old River Authorities and Water Boards took place in 1.974 with the formation of the major Regional Water Authorities of which two cover the greater part of the area considered in this review -Wessex Water Authority and South West Water Authority. Whatever opinion we may have about the effect of this reorganisation on our household expenses, one thing it has done is to make the organisations of sufficient size to employ the scientific backup to permit

proper hydrogeological investigations to be carried out. In the old days, water boreholes were sunk on a minimum of geological forethought, a pump test was carried out along with a chemical analysis and that was that, either production was started or it was not depending on the result. Little thought could be given to aspects such as total groundwater potential or optimum recovery. In the South West Water Authority the amount of water supply from groundwater is only a small proportion of the total, the remainder coming from rivers which are now, or are proposed to be, regulated by dams. But locally in East Devon, where most of the groundwater is found in the Triassic aquifer of the Bunter Pebble Beds and Otter Sandstone, supply is dependent upon groundwater. In the Wessex Water Authority there are again local variations in the proportion of supply coming from groundwater, although the overall importance of this source is much greater than in the South West. In the Dorset-Avon Division of Wessex the Chalk provides most of the supply and as an example of the type of work done in the 1970s, we have the excellent recently published map prepared jointly by geologists of the Institute of Geological Sciences and of the Wessex Water Authority (1979) to show Hydrogeology of the Chalk and Associated Minor Aquifers in the area. In the investigation of groundwater resources, a wide variety of geological techniques is brought to bear. Geophysical logging is a major tool introduced during the last few years. Flow logging for instance follows the movement of water in a borehole using conductivity measuring equipment to trace a slug of salt solution introduced into the hole. This shows up zones and rates of ingress or egress of water and can be compared directly with lithological and geophysical logs to indicate the main producing zones within a well. Referring back to the early part of the decade and the start of the more scientific approach to groundwater evaluation, a major contribution to the hydrogeological understanding of the East Devon aquifer was made by Sherrell (1970). The influence of Fred Sherrell in the advancement of knowledge in applied geology in the South West is widespread and far-reaching. Sherrell's paper on this area stemmed in part from his work on the influence of the quarrying of Bunter Pebble Beds at Blackhill Common, near Exmouth on the aquifer potential, the Common being part of the catchment area. Following a lengthy public inquiry, a compromise was reached whereby the quarrying company accepted some sterilisation of reserves with a stringent working and landscaping scheme including the development of lake areas to ensure as much recharge of the aquifer as possible. Further instances of the recurring conflict of interest between hydrogeology and quarrying were highlighted in the very important study "Quarrying in Somerset" (Somerset County Council, 1971) which heralded similar studies by other counties. This study showed the need for

the limestone quarrying industry and the water industry in the Mendip areas to get together to achieve the best possible balance in resource Conservation. Unfortunately at the end of the decade not much progress has been made towards the required research programme and this must now be more urgent for the 1980s, in good time before extensive quarrying below the water table becomes necessary. The Mendip area will be expected to supply increasing proportions of the constructional materials required in the south-east as gravel reserves in that area become progressively exhausted. Energy This review can not be considered complete without reference to energy. The South West has been disappoint-ingly short of energy sources although with the South Wales coal fields just across the sea this has not been too much of a hardship. The small and complex coalfield in the Bristol area cannot be worked economically by modern mining methods. Oil has been produced in a small way in Dorset for many years but there has been the recent major discovery at Wytch Farm near Corfe Castle and much activity with further exploration licences granted in West Dorset, East Devon and Somerset. The main Wytch Farm discovery has been in Triassic rocks below the older discoveries and now there are proposals to drill deeper at other small established sites, proposed production from Wytch Farm is 20000 barrels per day which is about 1 per cent of UK needs. The oil-bearing structure, which involves reversed pre-Chalk faulting to allow oil originating from Jurassic sediments to accumulate in the Triassic reservoir rock, extends eastwards into Poole Bay and further exploration may increase its importance. The potential of the offshore prospects around the South West has still not been followed up in detail and this is another question for the 1980s. As has been illustrated in the Aberdeen area and in Shetland, active off-shore exploration creates an upsurge in activity in the construction and related industries on-shore with repercussions for mineral production and engineering geology around the servicing areas of which the best possibilities in the South West seem to be Plymouth and Falmouth, the latter suffering at present from poor communications. Continuing to speculate about the future in energy, there has been much debate recently about the requirement for a new power station in the western end of this region and argument as to whether or not it should be nuclear or conventional. However, there also remains the possibility of a tidal barrage scheme in the Severn Estuary for which three possible sites have been suggested. Preliminary feasibility studies suggest that a barrage from Cardiff to

Weston could generate 10 per cent of this country's energy requirements by the year 2000 at a cost not vastly greater than the cheapest nuclear power stations for the same generating capacity - around £3000 million. Of course there are formidable environmental problems. Many might say however, that the environmental change would be for the better. Detailed feasibility studies will require engineering geological and sedimentological input. Constructional materials requirements will be large and have been variously estimated at 60-200 million tonnes for the fill material alone. Quarries in Mendip and South Wales could supply such quantities by stretching their capacity but there may also be some scope for using the estuary sediment or for the use of waste materials such as coal waste, china clay waste, or Hemerdon waste for instance, but transport costs and logistics are likely to be prohibitive. Conclusion This review brings together information on a wide range of projects, each having important geological involvement and in total illustrating the valuable contribution which geological studies have made to the overall prosperity of the region throughout the decade. Acknowledgements. Discussions with innumerable friends and colleagues throughout the 1970s contributed to this review. Special thanks are due to Keith Atkinson, Keith Beer, John Cowley, David Heath, Chris Keeler, Paul King, Andrew Meade, Fred Sherrell, Cliff Tubb, Ian White and to colleagues in the English China Clays Group for valuable assistance during preparation of the paper. References Freshney, E.C. 1970. Cyclical Sedimentation in the Petrockstow

Basin, Proc. Ussher Soc., 2, 179-89. Institute of Geological Sciences and Wessex Water Authority.

1979. Hydrogeological Map of the Chalk and Associated Minor Aquifers of Wessex. 1:100,000.

Ministry of Housing and Local Government. 1970. South West Economic Planning Region: Regional Information Map. 1:250,000.

Ripley, M.J. 1972. Slope Stability Problems in the China Clay Industry of South West England, Proc. Ussher Soc., 2, 405-8.

Sherrell, F.W. 1970. Some aspects of the Triassic Aquifer in East Devon and West Somerset. Q. Jl Engng Geol., 2, 255-286.

Sherrell, F.W. 1971. The Nag's Head Landslips, Cullompton By-Pass, Devon. Q. JI Engng Geol., 4, 37-73.

Somerset County Council. 1971. Quarrying in Somerset. 349pp. Vincent, A. 1972. Drilling Techniques in Ball Clays. Q. Jl

Engng Geol., 4, 241-47.

Read at the Annual Conference of the Ussher Society, January 1980

The Yes Tor Member of the Hangman Sandstone Group (North Devon) I.P. TUNBRIDGE

I.P. Tunbridge 1980. The Yes Tor Member of the Hangman Sandstone Group (North Devon). Proc. Ussher Soc., 5, 7-12. The Hangman Sandstone Group represents a Middle Devonian incursion of continental facies into the north Devon area. The main phase of continental deposition is represented by the fine-grained sheetflood sandstones and thin siltstones of the Trentishoe Formation (c.1,250m thick), with a provenance from South Wales. The highest beds of the Trentishoe Formation, termed the Yes Tor Member, include thick purple siltstones containing calcareous nodules of probable pedogenic origin. These siltstones are considerably thicker than any seen in the main body of the Trentishoe Formation and record a marked decline in the rate of sedimentation. The overlying coarse-grained sandstones and conglomerates of the Rawns Formation (148m thick) represent a short-lived episode of high-energy alluvial sedimentation, with angular exotic clasts derived from a nearby northerly source situated in the present Bristol Channel. The continental phase is terminated by the marine transgression of the Sherrycombe Formation. The Yes Tor Member marks not only the end of the main phase of continental deposition in north Devon, but also the termination of the Lower to early Middle Devonian episode of Old Red Sandstone sedimentation in southern Britain, heralding the mid-Devonian 'lull'. L P. Tunbridge, Geology Division, Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth PL4 8AA

Introduction The Hangman Sandstone Group (Tunbridge 1978a,b) is an incursion of continental and shallow marine sandstones and siltstones into the mainly marine sequence of the north Devon Devonian. The Group ties above the Emsian Lynton Beds (Simpson 1964) and below the Givetian Ilfracombe Slates (Whittaker 1978), and may therefore be regarded as being of Eifelian age. The Hangman Sandstone Group outcrops in a major anticlinal structure extending over much of northern Exmoor and along the coastline of north Devon and west Somerset (Fig. 1). Inland exposures are extremely poor, but excellent coastal outcrops permit a detailed litho-stratigraphy to be recognised, which can be broadly followed inland. The Group consists of five formations. The Hollowbrook Formation lies above the shallow-water marine Lynton Beds and consists of c.70m of shoreface sandstones and offshore heterolithic beds. The overlying Trentishoe Formation (c.1,250m thick) comprises continental sheetflood sandstones, thin floodbasin siltstones and laminated and desiccated ephemeral lake mudstones. The Trentishoe Formation terminates in the Yes Tor Member, a series of thick siltstones and sandstones which lies immediately beneath the Rawns Formation. The base of the Member is obscure, but on the coastal section east of Combe Martin (SS589484) a 16m-thick sequence comprising purple siltstones up to 7m thick together with

parallel-laminated and cross-bedded sandstones can be seen. The base is obscured by faulting at sea level, but high in the cliffs the sequence can be traced, passing downwards into typical Trentishoe sandstones. The total thickness of the Member is estimated to be 20m. The succeeding Rawns Formation consists of 148m of coarse sandstones and conglomerates with subordinate siltstones and contains angular exotic clasts, representing low sinuosity channel deposition, derived from a nearby northerly source. A marine transgression marks the base of the 90m-thick Sherrycombe Formation which represents estuarine or fan-delta conditions in a series of upwards-coarsening sequences. The Little Hangman Formation (c.100m thick) records more open marine conditions in a series of interbedded heterolithic grey mudstones and sandstones, passing upwards into the Wild Pear Slates (Whittaker 1978) at the base of the Ilfracombe Slates.

The Hangman Sandstone Group records continental Old Red Sandstone type deposition in the Trentishoe and Rawns Formations. This period of deposition was not continuous, as a pause in sedimentation occurred at the end of the Trentishoe Formation, marking an end to the main episode of clastic deposition in north Devon. This 'pause' is recorded in the Yes Tor Member, which marks a change in depositional style from the main body of the Trentishoe Formation, characterised by thick siltstones and the presence of pedogenic carbonate nodules.

The Trentishoe Formation The Trentishoe Formation is dominated by parallel-laminated fine- to medium-grained red and green sandstones. Individual beds are rarely more than 1.5m thick and many are sheet-like in geometry. Some sandstones have a thin basal intraformational conglom-erate, and may be capped by 5-10cm of ripple cross-laminated fine-grained to very fine-grained sandstone. The parallel-laminated sandstone beds are arranged in multistorey bodies which alternate with, and occasionally interfinger with, red and green siltstones (Fig. 2). These siltstones are rarely more than 50cm thick and some are associated with 3-10cm thick very fine-grained ripple cross-laminated and parallel-laminated sandstones. This sequence is interpreted as the product of high energy sheetflood sedimentation (Tunbridge, in press). Wholly parallel-laminated sands are rarely found in perennially flowing streams but are common in ephemeral deposits (McKee and others 1967). The interbedded and interfingering siltstones and very fine-grained sandstones record waning flow and distal flood sedimentation (Scott and others 1969).

Thicker sequences of siltstones, mudstones and very fine-grained sandstones occur in parts of the Trentishoe Formation. In this facies, interbedded and interlaminated siltstone, mudstone and sandstone occur on a scale of 5cm or less in thickness. This thinly layered and laminated sequence is remarkable for the abundance of desiccation and water escape structures which disrupt almost every bed (Fig. 3), bearing similarities to parts of the Lower Devonian Ringerike Group of Norway (Whitaker 1964), and representing deposition in an ephemeral lake subject to repeated wetting and drying.

Figure 1. Sketch map of the outcrop of the Hangman Sandtone Group (stippled) showing localities referred to in the text Petrologically, the sandstones of the Trentishoe Formation are lithic arenites, containing rock fragments of sandstone, tuffs, rare acid lavas, phyllites and schists. Feldspars are a minor component, comprising less than l0 percent and including microcline, braid perthite, plagioclase (An55-75) and altered orthoclase. This composition matches closely that of the Brownstones (Allen 1974; Tunbridge 1978a), which form the highest Lower Old Red Sandstone in South Wales and the Welsh Borderlands. Allen (1974) concludes that the source for the Brownstones lay in areas of North Wales composed of Silurian and Ordovician sedimentary and high level igneous rocks, together with some reworked metamorphic elements. The Trentishoe Formation could also have had a source in North Wales which persisted into the Middle Devonian. but alternatively the sandstones could be from a source of reworked Brownstones. The latter possibility is the more feasible, as the Middle Devonian is marked by a period of erosion and uplift in South Wales (Jones 1956; Allen 1979), with the top of the Brownstones being eroded prior to the deposition of the Upper Old Red Sandstone. The major part of the Hangman Sandstone Group may well represent the product of mid-Devonian erosion in South Wales. The Yes Tor Member On the north Devon coast, the Trentishoe Formation passes to the Rawns Formation at Yes Tor (SS589484), a promontory extending into the sea 2km east of Combe Martin. This locality is surrounded by sea at all states of

Figure 2. A. Sedimentological log of a typical sequence in the main part of the Trentishoe Formation at Foreland Point (SS755513), showing parallel-laminated sandy sheetflood sediments alternating with interbedded sandstones and siltstones of distal/ marginal flood deposits. B. Log of the exposed sequence of the Yes Tor Member at Yes Tor (SS589484), from low water mark to the base of the Rawns Formation.

Figure 3. Desiccated and water escape structures in interbedded and laminated sandstones and mudstones of the Trentishoe Formation. Despite clear evidence for episodes of desiccation, no calcretes have been found in this facies. The coin in the centre of view is 3cm in diameter. the tide and may only be reached by boat. The passage is marked by a major facies change in the highest beds of the Trentishoe Formation. These beds are termed the Yes Tor Member of the Trentishoe Formation and are immediately overlain by the Rawns Formation. Owing to the faulted nature of the coastline, the base of the Yes Tor Member cannot be studied at sea level. In the cliffs above Yes Tor, the transition from the Yes Tor Member to the main body of the Trentishoe Formation can be traced, permitting an estimate of 20m to be made for the thickness of the Member. In contrast to the sandstone-dominated main part of the Trentishoe Formation, the Yes Tor Member is dominated by purple siltstones. In the highest bed of the Member, the siltstones are over 7m thick (Fig. 2). This thickness is remarkable in that it is much greater than any siltstone in the underlying parts of the Trentishoe Formation. Furthermore, nodular micritic carbonate is contained in the upper parts of the siltstone, a feature very rarely found in underlying beds. The nodules are up to 3cm wide and constitute up to 5 per cent of the rock in the highest parts of the siltstone, decreasing downwards in

abundance to less than 1 per cent of the rock at a level 2m below the top of the bed. The nodules occur in bands and sometimes show signs of poorly developed pseudo-anticlines (cf. Allen 1973). The nodules seem to resemble the pedogenic carbonate glaebules (calcretes) of semiarid soil profiles (Gile and Hawley 1966). Clasts of eroded calcretes occur in the basal conglomerate of the overlying Rawns Formation. The siltstones of the Yes Tor Member cap fine-grained sandstones forming a fining-upwards unit. Three Sandstone facies are recognisable: a) Parallel-laminated sandstones in beds up to 2m thick, with an even, closely-spaced horizontal lamination. Primary current lineation was not observed, due to the lack of suitable bedding plane exposures. A siltstone-clast conglomerate 10cm thick lies at the base of each interval, above an irregular erosion surface. Limited lateral exposure does not permit the determination of sand body geometry. The facies is interpreted as upper flow-regime plane bed deposition, with the intraformational conglomerate representing high energy erosion from underlying silts.

b) Cross-bedded sandstones, with l-l.5m thick sets of planar to slightly concave foresets. Siltstone clasts lie along some foresets and above basal erosion surfaces. The sets have planar tops and bases. Palaeocurrent directions are hard to measure owing to the difficulties of close access, but are unidirectional, southerly and westerly directed. The facies is interpreted as the product of migrating sandwaves or small bars. c) Ripple cross-laminated very fine-grained sandstones in beds 0.3-0.6m thick, containing trough-shaped 4-6cm-thick sets. Palaeocurrents could not be measured from this facies. The facies is interpreted as lower flow-regime migrating ripple trains. The sediments of the Yes Tor Member are finer grained and more thickly bedded than the sheetflood sandstones and siltstones of the underlying Trentishoe Formation, and are quite distinct from the thinly bedded desiccated mudstone facies of the Trentishoe Formation. The Member clearly represents a major change in depositional style. The parallel-laminated and cross-bedded sandstones (Facies a and b) are interpreted as channel sands overlain by thick floodbasin silts. The ripple cross-laminated sandstones (Facies c) are interbedded with the overbank siltstones and are interpreted as overbank flood sands (Jahns 1947). The fining-upwards sequence developed in the member is reminiscent of a meandering stream sequence (Allen 1970), but no lateral accretion surfaces have been observed within the sequence and a meandering stream origin cannot be conclusively supported. However, the large amounts of fine-grained sediments present in the Member would seem to point to a change in alluvial style from the sandy sheetfloods below as there is a frequent association between meandering channel deposition and high suspended loads (Schumm and Kahn 1972). The presence of thick calcrete-bearing siltstones in the Member suggests a reduction in sediment-supply and basin gradient compared to the high energy sandy deposition of the main part of the Trentishoe Formation, giving a quiescent period marked by a change in fluvial regime. The change is reinforced by the presence of calcretes, which suggest extended periods of non-deposition (Leeder 1974) at the end of Trentishoe times. Changes in alluvial regime are well known from the Recent and late Quaternary and may reflect tectonically induced changes in sediment supply from the source area (Schumm 1969). In the case of the Yes Tor Member, the change of silt-dominated deposition may reflect a decrease in erosion rates in the South Wales area. The Rawns Formation Overlying the Yes Tor Member are gravel sheets at the base of the Rawns Formation, This Formation is characterised by thin siltstones, coarse- grained sandstones, and conglomerates containing

angular exotic clasts. The clasts are up to 60mm wide and include tuffs, quartz-feldspar porphyry, microgranite, quartzite, lithic arenite, metaquartzite and vein quartz. Many clasts are highly angular, and their source area could not have been far distant. Palaeocurrents from cross-bedded sandstones indicate a northerly provenance (vector mean 1790). The clasts are compositionally distinct from the rock fragments in the sandstones of the Trentishoe Formation, and a separate source is indicated. A likely candidate for this nearby, northerly source could be the Lower Palaeozoic rocks which at present lie at shallow depths beneath the northern part of the Bristol Channel (Brooks and others 1977; Brooks and AI-Saadi 1977). The Rawns Formation marks a return to coarse-grained sedimentation following the fine-grained interlude of the Yes Tor Member, hut with derivation from a more localised source than the Trentishoe Formation. The Rawns episode was relatively short-lived, since the 148m of the Formation is terminated by a marine transgression at the base of the Sherrycombe Formation, marking the return to marine conditions. Discussion The Eifelian of north Devon is represented by an incursion of Old Red Sandstone facies. Throughout much of the European succession the Eifelian marks a return to argillite deposition, interpreted by House (1975) as a marine deepening. At the same time, major transgressions occurred across the Russian platform and in North America (House 1975). The local regression in north Devon seems anomalous, but a cause may be found in the mid-Devonian uplift and erosion of the Lower Old Red Sandstone in South Wales (Allen 1979). It was from this source that floods of detritus were derived for the Trentishoe Formation, the rate of supply from the uplifted Welsh mountains being so great as to locally stem the marine advance. The main Trentishoe phase ended with a change in alluvial style to the silty Yes Tor Member, marked by an increase in the deposition of fine-grained suspended sediments and a relative slowing of sandy sedimentation, With calcretes indicating a break in deposition prior to the clastic influx of the Rawns Formation. The Yes Tor lull may be regarded as the termination of the main clastic episode which produced the Hangman incursion of continental sediments into north Devon. In a wider context, the lull may represent the end of the main southern British Lower to early Middle Devonian clastic phase (Allen 1979) and herald the mid-Devonian argillite phase (House 1975). The succeeding Rawns Formation is, in comparison to the Trentishoe Formation, a minor depositional .event, derived from a nearby and short lived source, terminated by the marine transgression of the Sherrycombe Formation.

Acknowledgements. Field work was carried out during the tenure of a University of Reading Research Studentship, the receipt of which is gratefully acknowledged. Mr K. Russell kindly read a draft of this paper, which was stimulated by discussions with Professor J.R.L. Allen, F.R.S. References Alien, J.R.L. 1970. Studies in fiuviatile sedimentation. A

comparison of fining-upwards cyclothems with special reference to coarse member composition and interpretation. J. sedim. Petrol., 40, 298-323.

Allen, J.R.L. 1973. Compressional structures (Patterned ground) in Devonian pedogenic limestones. Nature, 243, 84-86.

Allen, J.R.L. 1974. Source rocks of the Lower Old Red Sand-stone; exotic pebbles from the Brownstones, Ross-on-Wye, Hereford & Worcester. Proc. Geol. Assoc., 85, 493-510.

Allen, J.R.L. 1979..Old Red Sandstone facies in external basins, with particular reference to southern Britain. In House, M.R., Scrutton, C.T., and Bassett, M.G. (eds). The Devonian System. Spec. Pap.'Palaeont., 23, 65-80.

Brooks, M., and AI-Saadi; R.H. 1977. Seismic refraction studies of geological structure in the inner part of the Bristol Channel. Q. ,Il. geol. Soc. Lond., 133,433-446.

Brooks, M., Bayerley, M. and Llewellyn, D.J. 1977. A new geological model to explain the gravity gradient across Exmoor, north Devon. Q. Jl. geol. Soc. Lond., 133, 385-393.

Gile, L.H. and Hawley, J.W. 1966. Periodic sedimentation and soil formation on an alluvial fan piedmont in southern New Mexico, Proc. Soil. ScL Soc. Am., 30, 261-268.

Holwill, F.J.W., House, M.R., Gauss, G.A., Hendriks, E.M.L. and Dearman, W.R. 1969. Summer (1966) field meeting in Devon and Cornwall. report of the directors. Proc. Geol. Assoc., 80, 43-62.

House, M.R. 1975. Facies and time in Devonian tropical areas. Proc. Yorks. geol. Sbc., 40, 233-288.

Jahns, R.M. 1947. Geologic features of the Connecticut Valley, Mass., as related to recent floods U.S. Geol. Surv Water Supply Pap., 996, 158pp.

Jones, O.T. 1956. The geological evolution of Wales and the adjacent regions. Q. ,Il. geol. Soc. Lond., 111,323-351.

Leeder, M.R. 1975 Pedogenic carbonates and flood sediment accretion rates: a quantitative model for alluvial arid-zone lithofacies. Geol. Mag., 112, 257-270.

McKee, E.D., Crosby, E.J. and Berryhill, H.C. 1967. Flood deposits, Bijou creek, Colorado, June 1965. J. sedim. Petrol., 37, 829-851,

Schumm, S,A. 1968. River adjustment to altered hydrologic regimes-Murrumbidgee River and palaeochannels, Australia. U.S. Geol. Surv. Prof. Pap., 352-C, 31-68.

Schumm, S.A. and Kahn, H.R. 1972. Experimental study of channel patterns. Bull. geol. Soc. Am., 83, 1755-1770.

Scott, A.J., Hoover, R.H. and McGowen, ,J.H., 1969. Effects of Hurricane "Beulah" 1967 on Texas coastal lagoons and barriers. Lagunas Coasteras. Un Simposio. Mem. Simp. lnkm. Lagunas Costeras. UNAM-UNESCO. 221-236.

Simpson, S. 1964 The Lynton Beds of North Devon. Proc. Ussher Soc., 1, 121-122.

Tunbridge, I.P. 1978a. The sedimentology of late Lower and early Middle Devonian rocks of the Bristol Channel region, southern England. Unpubl. Ph.D thesis, Univ. of Reading, 440pp.

Tunbridge, I.P. 1978b. Lynton Beds and Hangman Sandstone Group. In Scrutton, C.T. (ed). International Symposium on the Devonian System (P.A.D.S. 78) September 1978. A field guide to selected areas of the Devonian of south-west England. The Palaeontological Association, 8-13.

Tunbridge, I.P. In press. Sandy high energy flood sedimentation - some criteria for recognition, with an example from the Devonian of S.W. England. Sedim. Geol.

Whittaker, A. 1978. Ilfracombe Slates. In Scrutton, C.T. (ed). International Symposium on the Devonian System (P.A.D.S. 78) September 1978. A field guide to selected areas of the Devonian of south-west England. The Palaeontological Association. 13-18.

Whitaker, J.H.McD. 1964. Mudcrack diapirism in the Ringerike Sandstone of southern Norway. Norsk. geol. Tidsskr., 44, 19-30.

Stromatoporoid morphotypes of the Middle Devonian Torbay Reef Complex at Long Quarry Point, Devon STEPHEN KERSHAW ROBERT RIDING

S. Kershaw and R. Riding 1980. Stromatoporoid morphotypes of the Middle Devonian Torbay Reef Complex at Long Quarry point, Devon. Proc. Ussher Soc., 5, 13-23.

Stromatoporoids dominate the Middle Devonian reef facies at Long Quarry Point, Torquay, in south-east Devon and constitute approximately 18% of the rock volume. The only other conspicuous unfragmented skeletal constituents are rugose and tabulate corals. Abundant crinoid fragments dominate the coarse sand-gravel grade matrix. The section, which is estimated to be 150m thick, shows a distinct transition from laminar stromatoporoids in the lower part of the section, through domical forms in the middle, to bulbous forms near the top, The raggedness (presence of sediment invaginations), volumetric importance, size, and number of stromatoporoids decrease upwards through the section while disorientation of specimens increases. The laminar and domical forms are large (maximum dimensions 5.5 and 1.7m respectively), in place, and commonly have ragged margins. Their size and abundance would have had a major influence upon sedimentation in the area and we regard the lower 50m of the section, dominated by larger laminar and domical stromatoporoids constituting approximately 20% of the rock volume, as representing reef deposits. The bulbous forms are small (maximum dimensions 30cm), 'smooth', and Commonly fallen or overturned and constitute a smaller proportion (approximately 10%) of the rock volume in the facies they dominate near the top of the section. The section as a whole represents a shallowing-up sequence from upper fore-reef and reef crest (laminar and domical) to back-reef (bulbous) zones.

Stephen Kershaw, West London Institute of Higher Education, Borough Road, Isleworth, Middlesex TW7 5DU; Robert Riding, Department of Geology, University College, Cardiff CFI IXL

Introduction The purpose of this study is to apply a recently developed technique of stromatoporoid shape description to one of the main outcrops of the Torbay Reef Complex of Middle Devonian age in south-east Devon. The aims are two-fold; firstly to test and refine the morphotype scheme (Kershaw and Riding 1978), and secondly to elucidate the pattern of stromatoporoid shape variation through the sequence. Stromatoporoids are prominent components of middle Palaeozoic reef limestones, but their study has been limited by the lack of taxonomic detail observable by the naked eye, thin sections usually being needed for identification. By measuring shape parameters of the skeletons we aim to describe their external morphology with sufficient precision to provide a firm basis for their comparison and interpretation. The Torbay Reef Complex is well suited to this purpose since it is dominated by a variety of stromatoporoid morphotypes, and the section at Long Quarry Point (Fig. 1) in particular shows a remarkable development of large, closely spaced specimens (Scrutton 1977b, p.189).

We recognise a clear trend in morphotypes through the section from laminar forms near the base, to domical forms in the middle, and bulbous forms near the top. Because the technique we use is based on measurement of specimens, we are able to present the results graphically and to compare them, with a fair degree of objectivity, with results obtainable from other localities, thereby placing the study and description of stromatoporoid morphotypes on a firmer basis than has hitherto been achieved. Methods We recognise four basic stromatoporoid shapes: laminar, domical, bulbous and dendroid; "ragged" and "smooth" varieties of these reflect the presence or absence of sediment invaginations at the margin of the coenosteum. These terms are used to describe broad categories of forms but the morphology of a particular specimen is recorded by measuring the basal, vertical and diagonal proportions of vertical sections through the skeleton

Figure 1. a) Outcrops (black) of Givetian limestone at Torquay and location of Long Quarry Point. b) Plan of Long Quarry Point with positions of the measured transect segments. The strata appear to dip NNW at approximately 400. c) Segments (numbered) and intervening poorly exposed areas (black) corrected for dip and showing true stratigraphical thickness.

Figure 2. Stromatoporoid parameters and features recorded for each specimen encountered along the transect segments. B, basal length; V, vertical height; D, (average of DI and D2), diagonal distance at angle θ from V; RV (average of RV 1 and RV2), height to which raggedness extends from base; R H(sum of R H I and RH2), lateral extent of raggedness from margin; A, way-up of stromatoporoid relative to bedding.

(Fig. 2), These dimensions can be plotted on triangular arrays which provide a simple visual display of the data. These techniques are described in detail by Kershaw and Riding (1978).

At the same time as this information is being collected it is also useful to measure the density of stromatoporoids and this was done by noting the position of each specimen along a line transect. A data sheet (Fig. 3) was used to record these various features of the stromatoporoids. The transect was made normal to the strike of the section and continuous recording was made of stromatoporoid spacing and dimensions on five "clear" (i.e. not covered by vegetation, water or soil) segments of the transect (Fig. 1). Details were not recorded from the intervening more or less covered segments. Each stromatoporoid encountered along the line was measured for B, V and D (Fig. 2). It should be noted that maximum values of these features were taken, such that every specimen encountered on the transect was included in the data collection, but the parameters were measured at their maximum points on the specimen, not necessarily at the point where the transect crossed it.

Since the section is dipping approximately 400o NNW, the parameters V and D will appear exaggerated. Conse-quently these observed values of V and D (Vo and Do respectively) required corrections to the true V and D before analysis could take place. However, B needed no correction (see Appendix). The triangular arrays (Figs. 4 and 5) were plotted from corrected data only.

In order to provide an estimate of the proportion of stromatoporoid to matrix, the thicknesses of stromatoporoids along the transect were also recorded. In addition, the degree of raggedness, if developed, was measured (Figs. 2 and 3), together with the orientation of the stromatoporoids relative to the top of the section (deviation of V from the line of the transect).

The fundamental bias of this method is that although vertical sections through coenostea are measured (or corrections made to oblique sections) only a small proportion of these randomly generated natural sections will pass through the true centres of skeletons. Sections displaced towards the margin of a coenosteum will inevitably tend to give reduced values of B, V and D in all stromatoporoid forms because stromatoporoids are usually thickest in the centre and crudely circular in plan view. Unfortunately, these effects do not tend to cancel each other out when the values are converted into ratios because of the shape variations involved. In domical forms V and D will generally be reduced by this effect more than B, but in laminar forms B will suffer most. Consequently the co-ordinates of domical forms on the triangular arrays will be slightly displaced towards the basal pole, while the position of laminar forms will be shifted away from it. However, since these biases will always be present in data collected on rock surfaces in

Figure 3. Field data sheet headings. Posn, distance of specimen from base of transect; No., number of the specimen encountered; A, attitude; B, basal length; V, vertical height; D, diagonal distance; R,S, ragged or smooth (if ragged then RV and RH are recorded, see Fig. 2, if smooth then "s" is written in RV column); TT, transect thickness, the distance occupied by the specimen on the line of the transect. Compare Fig. 2.

this way, we believe they will not significantly affect comparisons between data from different facies and' localities provided that the technique is consistently applied. The only way to measure maximum values of B, V and D is to extract the stromatoporoids from the matrix and slice them through the centre. Not only is this latter procedure time consuming, it is extremely difficult in firm matrices, and is unlikely to be widely applied. But if it is performed then the slight difference in the resulting Plots anticipated here should be looked for and the difference in method should be taken into consideration when making comparisons with data collected in the way we have used here. Long Quarry Point Long Quarry Point is a promontory on the eastern side of Torquay, where Givetian limestones have been recognised since Ussher (1903). The reefal nature of the rock has long been suspected, and contrasting attempts have been made to include the locality in environmental interpretations within the framework of the Middle Devonian sediments of south Devon (e.g. Dineley 1960; Braithwaite 1967). The most recent of these (Scrutton 1977b) distinguishes a mosaic of facies within a major carbonate unit, the Torbay Reef Complex, and recognises a stromatoporoid barrier reef in the Torquay area which delimits back-reef environments occurring farther to the north. Although the term reef has been applied with caution to particular outcrops the Long Quarry Point section represents one of the most impressive developments of in-place skeletons in the area, and we concur with Scrutton (1977b, p. 172) in regarding it as a reef deposit. This outcrop shows a transition to back-reef facies at the top of the section (Scrutton 1977b. p.185). The base of the reef is seen to the south, near Redgate Beach (Scrutton 1978, p.35). The stromatoporoid morphotypes are so clearly displayed that the section provides an ideal

Figure 4. Stromatoporoid morphotypes from Long Quarry Point plotted according to transect segment (1-5). Inset shows the shapes represented by different points on the triangular array. For 1, n=32; 2, n=27; 3, n-26; 4, n-6; 5, n-4. Total 95 specimens.

opportunity to analyse and display the variation in shapes throughout the section using the parameterization scheme. Although the skeletal fabrics are often recrystallised in thin section, the fossils appear well preserved in the field, and the stromatoporoids can be easily distinguished from the other fossils, notably tabulate corals. Long Quarry Point is the best exposure of the Walls Hill Limestone (Scrutton 1977b, pp. 171-173). Scrutton describes the locality in some detail, noting the dip of the beds at about 450, together with the transition of stromatoporoid shapes from "sheet-like, tabular" (laminar of Kershaw and Riding 1978) forms at the base, passing through "regular conical" (domical) to "irregular" and "rounded" (bulbous) coenostea at the top. The Spar-filled cavities in the lower part of the sequence were noted by Braithwaite (1967, p.304). These cavities commonly occur beneath stromatoporoid coenostea, but the fact that they may cross-cut primary structures led Scrutton (1977b, p. 172) to regard some of them as being of secondary origin. They become less common higher up the sequence and the density of fossils decreases as does stromatoporoid size (Scrutton 1977b, p. 172). Scrutton notes that the sediment is coarsest in the central part of the sequence and that tabulate corals are more common, with Occasional intergrowth between fasciculate corals and stromatoporoids ("caunopora"). The top of the sequence shows disorientated stromatoporoids in a finer grained crinoidal sediment, to the exclusion of almost all other fauna. Results The five recorded segments of the section contain a total of 95 complete stromatoporoids together with some broken specimens. Volumetrically, all stromatoporoid skeletal material occupies, on average, 18% of the section. The stromatoporoids vary considerably in size; laminar and domical forms attain 5.5m and 1.7m respectively in their maximum dimensions, while bulbous forms are much smaller, generally less than 0.25m (Fig 7). The shapes are also highly variable (Fig 4); a clear trend of shapes from laminar through domical to bulbous is shown. Figure 4 combines smooth and ragged varieties, but the ragged forms, which are fewer and represent 20% of the total number of specimens in the section, are displayed in Fig. 5. Ragged forms are most abundant in segments I and 2, in forms which plot out towards the basal corner of the array, and consequently whose height is low. Sedimentation would affect these flatter (laminar to low domical) shapes to a greater degree than higher domical or bulbous forms and this accounts for the degree of raggedness which they show; the cause being simply that deposition of sediment on the margins of these forms caused invaginations of the skeleton.

Figure 5. Plot of ragged stromatoporoid specimens from transect segments 1-3 at Long Quarry point. No ragged forms were recorded from segments 4 and 5. For I, n=10; 2, n=7; 3, n=2. Total 19 specimens. Figure 6 illustrates the contrasts in stromatoporoid shapes between the five segments of the section and summarises abundance and raggedness variation together with the orientation of the coenostea. Data are presented as average values for each segment; further subdivisions are not particularly useful due to fairly small sample sizes in segments 4 and 5, and to the presence of relatively large gaps between segments where no data were collected. The density of stromatoporoid specimens is shown by the average number per metre in each segment, and presents a clear trend of decreasing numbers towards the top of the section (Fig. 6, lowest histogram). The abundance of stromatoporoids in a volumetric sense, taken as a percentage per metre of transect length (from the transect thickness, "T.T." column in Fig. 3), is shown for all the material present (Fig. 6, total stromatoporoid material) and this demonstrates a reduction in the importance of stromatoporoids near the top of the section. The sparser coral fauna was not measured in any detail, but corals are common in the middle part of the sequence (segment 3) where intergrowth of stromatoporoids and syringoporid corals occurs. The amount of raggedness is expressed in terms of ratios in the top three histograms of Figure 6; one demonstrates that the ratio of the numbers of individual ragged: smooth stromatoporoids decreases upwards, showing that stromatoporoids become less affected by sedimentation towards the top of the sequence. The remaining two plots treat ragged specimens only and reflect the actual degree of raggedness in each specimen; RV/V expresses the vertical extent of raggedness and RH/B expresses the horizontal extent of raggedness in each specimen, taken

Figure 6. Shape, orientation and abundance of stromatoporoids from Long Quarry Point plotted according to transect segments. From top to bottom: triangular displays of morphotypes, note trend from laminar to bulbous forms along transect; rose diagrams of specimen orientation, note increasing displacement along transect; raggedness (RH and RV) with respect to basal and vertical dimensions respectively, and proportions of ragged to smooth forms, note abrupt decline of raggedness near middle of transect; percentage of stromatoporoids occupying transect, note overall decline in importance towards top of section (to right); numbers of individual stromatoporoid specimens encountered, note decline up-section. Incomplete (fragmented) specimins were not included in the morphological analysis, but they are included in the “total stomatoporoid material” which reflects the volume of both broken and whole specimens in the section.

as an average for each segment. The histograms show that raggedness, as a whole, decreases up the sequence and that not only is there a greater number of smooth specimens towards the top, but that the ragged forms also become less ragged. Finally, Figure 6 also shows the attitudes of complete individual stromatoporoids expressed in partial rose diagrams, where it is obvious that stromatoporoids become more disorientated towards the top of the sequence. The maximum measured dimensions of laminar, domical (basal parameter) and bulbous (vertical parameter) stromatoporoids are shown in Figures 7 (entire sequence) and 8 (by segments), illustrating the smaller sizes of bulbous compared with domical and laminar forms at Long Quarry Point and the way in which overall stromatoporoid size decreases up through the sequence. In summary, the principal trends in shape and size variation are clearly sequence-related and show that laminar, domical and bulbous forms in turn successively dominate the subfacies encountered from base to top of the section. Abundance (both number of individuals and total volume), raggedness and size of stromatoporoids all decrease upwards, whereas disorientation of specimens increases upwards. Several of these factors are closely inter-related (see next section): both size and raggedness are greatest in laminar and domical forms, smoothness and disorientation are greatest for bulbous forms.

Figure 7. Size-frequency distributions of laminar, domical and bulbous morphotypes at Long Quarry Point (using B for laminar and domical forms, and V for bulbous). Discrimination between these morphotypes is based on Kershaw and Riding 1978. fig. 11.

Figure 8. Average dimensions (either B or V) for stromatoporoid morphotypes in each segment of the transect at Long Quarry Point. Note that bulbous forms are relatively small and occur near the top of the sequence. Laminar forms are large and occur near the base. Domical forms are intermediate in both respects. Discussion The parameterization method we have applied at Long Quarry Point allows a more detailed description of the stromatoporoids than has hitherto been possible. The measurement and display techniques provide means for accurately comparing morphotypes within and between outcrops. Our main aims in this study have been to test the methods through application and to describe the morphotypes present, hut this documentation leads inevitably to some discussion of the possible ecological and environmental implications of the changes recognised in the sequence. Variations in morphology The succession of morphotypes, from laminar to domical to bulbous forms, up through the sequence is a clear feature of the Long Quarry Point Outcrop, but it is accomplished gradually, as shown by the fairly broad shape spectrum in each segment recorded (Figs. 4 and 6), reflecting relatively gradual environmental changes through time. Morphotype variation is accompanied by reduction in a) raggedness, b) volumetric importance, c) size, d) number of stromatoporoids and increase in e) disorientation of specimens upwards. Some of these changes appear to be directly related to morphology, especially size and disorientation, and also raggedness. These size data (Fig 7) confirm our view (Kershaw and Riding 1978, p.234) that bulbous forms are generally smaller than laminar and domical forms. They also are particularly susceptible to overturning since they have small basal dimensions. Consequently, because bulbous forms dominate the upper

part of the sequence, coenosteal size is reduced there, and more specimens are overturned. We have also noted before (Kershaw and Riding 1978, table 1) that bulbous forms tend to be smooth rather than ragged. Therefore, upward decrease in raggedness through the sequence can also be ascribed to the progressive increase in abundance of bulbous forms and the concomitant decline of laminar and domical forms. But even in these latter forms, which readily develop raggedness (Fig. 5) due to covering of the margin of the coenosteum by sediment and its subsequent recolonisation by overgrowth from the middle of the stromatoporoid, raggedness becomes reduced as the middle of the Long Quarry Point sequence is approached (top two histograms, Fig. 6). This suggests that a general reduction in either the sedimentation rate or the lateral movement of sediment may have occurred at this time. However, caution needs to be applied in making this kind of deduction because other factors, such as the size of the stromatoporoids, may be important. Small individual coenostea may have been completely covered by sediment, preventing recovery, and preserving them as smooth, non-ragged forms, whereas larger specimens might not have been completely covered and would have recolonised marginal areas to produce ragged coenostea. Consequently size, as well as shape, could be significant in determining whether raggedness might develop, quite apart from sedimentation rates, and the overall patterns of size and shape distribution can be used to try to disentangle some of these inter-related factors. Figure 8 shows that the sizes of laminar and domical forms are reduced in segments 2 and 3 and, consequently, the general coenosteal size reduction up through the sequence shown in Figure 6 is not wholly due to the incoming of bulbous forms. It is therefore possible that rates of sediment accumulation in the middle part of the section were high enough to inhibit the development of large coenostea even though this conclusion is at first sight quite the reverse of that indicated by the decline of raggedness in this part of the sequence.

Ecology and environment The possible explanations of trends in stromatoporoid size and shape considered above show significant inter-relationships in coenosteal features, but they raise problems with respect to actual controls of stromatoporoid morphology. Indeed it is a general feature of stromatoporoid ecology that although broad patterns of morphotype distribution with respect to facies can be discerned, it is difficult to account for them in any detail. Although the information we have collected from Long Quarry Point does not elucidate specific controls of stromatoporoid morphology, it does help to clarify both the reefal nature of the sequence and its facies trend.

The reduction in total volume of stromatoporoids in the upper part of the section (Fig. 6) reflects an important facies change from laminar and domical stromatoporoid-

dominated substrates below, to gravels and sands with scattered bulbous forms above. In the lower and middle parts of the section the volume (approximately 20%) of stromatoporoid skeletons would have had significant effects in stabilising sediment and in providing hard substrates for other sessile organisms to attach to. These are reefal characters and support Scrutton's (1977b, p. 172 and Figs. 7, 8) view that the section represents part of a barrier reef facies. The upper part of the sequence is certainly different; Scrutton (1977b, p.172) regards it as "adjacent to, or on the flanks of, the crest of the structure" and it seems reasonable to consider it as back reef. However, Scrutton's (1977b, p.172) interpretation of the lower, laminar stromatoporoid-dominated part of the section, as reflecting a substrate "colonised by tabular coenostea in relatively quiet conditions" is questionable on the grounds that the matrix is coarsely bioclastic and that stromatoporoids are actually at their most abundant at this level in terms of both volume (25%) and numbers of individuals (Fig. 6). We prefer to argue that this laminar sub-facies is the upper fore-reef and that the domical sub-facies above it is the reef crest zone. If this interpretation is extended to the Berry Head area near Brixham, 8km south of Torquay, it implies that the "tabular stromatoporoid-crinoid' facies recognised there by Mayall (1979, p. 178) did reach the wave zone and helps to explain the lateral development of restricted Amphipora facies. Mayall (1979, p. 178) recognises progressive shallowing in the Berry Head sequence, and the Long Quarry Point section, although slightly older (Scrutton 1977a, Fig. 2), also shows a shallowing-up succession of reef-related zones from laminar to domical to bulbous stromatoporoid facies which may be overlain by Amphipora facies in the Walls Hill area (Scrutton 1977b, p.172).

Methods This study is the first application of the parameterization scheme to Devonian stromatoporoids. Although Devonian morphotypes can show complicated variations on basic forms (Kershaw 1979, Chapter 2) we have not experienced any particular problems in applying the method during this study. The only modifications which proved to be necessary are those correcting for the oblique sections encountered in these dipping strata (see Appendix). It is certainly easier to process data from vertical sections of stromatoporoids, but nevertheless, the correction techniques which we describe here are straightforward and simple to apply, except in disorientated specimens, and these are difficult to deal with in any case.

Conclusions 1. The Givetian limestones of Long Quarry Point, south-east Devon, show a gradual but distinct upward transition from laminar to domical to bulbous stromatoporoid facies through a 150m thick sequence. 2. Stromatoporoids constitute, on average, approximately 18% by volume of the total sequence, and are associated with rugose and tabulate corals in the middle part of the section, and with crinoid debris throughout. 3. The size, volumetric and numerical importance, and raggedness of the stromatoporoids are greatest in the lower part of the section and decrease progressively upwards. Disorientation of specimens increases upwards. Several of these features, notably size, raggedness and disorientation, are directly related to morphotype: laminar and domical forms tend to be large (up to 5.5m and 1.7m respectively across) and ragged; bulbous forms are smaller (30cm maximum) smooth, and commonly disorientated. 4. The lower 50m of the section, dominated by laminar and domical forms constituting up to 25% of the rock volume, is regarded as reef facies. The upper 100m, with mainly domical and bulbous forms comprising 10-15% of the rock volume, is interpreted as a transition to back-reef facies, and the overall facies trend can be seen as a shallowing-up sequence. 5. A simple correction method can be linked with the basic parameterization scheme (Kershaw and Riding 1978) and readily allows data collected from oblique (non-vertical) sections of stromatoporoids to be processed (see Appendix). Acknowledgements. Data collection was carried out with the assistance of G.J.J Clarke. R.M. Coventry, T.H.J. Gosden, D.W. Hughes, and M.J. Smith. whose help and stimulating discussion are gratefully acknowledged. Riding was first introduced to Long Quarry Point by Colin Scrutton who also criticised the manuscript and provided helpful comments. References Braithwaite, C.R.J. 1967. Carbonate environments in the

Middle Devonian of South Devon, England. Sediment. Geol. 1, 283-320.

Dineley, D.L. 1960. The Devonian System in South Devonshire. Fld. Stud., 1, 121-140.

Kershaw, S.J. 1979. Functional and environmental significance of skeletal morphology in stromatoporoids. Unpublished Ph.D. thesis, University of Wales, 408pp.

Kershaw, S. and Riding R. 1978. Parameterization of stroma-toporoid shape. Lethaia, 11, 233-242.

Mayall, M.J. 1979. Facies and sedimentology of part of the Middle Devonian limestones Of Brixham. South Devon. England. Proc. Geol. Assoc., 90, 171-179.

Scrutton, C.T. 1977a. Reef facies in the Devonian of eastern south Devon, England. Mém. Bur. Rech. géol. minier. 89, 125-135.

Scrutton, C.T. 1977b. Facies variations in the Devonian lime-stones of-eastern South Devon. Geol. Mag., 114, 165-193.

Scrutton, C.T. 1978, Eastern South Devon. In: Scrutton C.T., ed. Devonian of south-west England, Field guide, Intern. Symp. Devonian System 1978, pp.2749.

Ussher, W.A.E. 1903. The geology of the country around Torquay. Mem. geol. Surv. Engld & Wales, 149pp.

Appendix Corrections to data from oblique sections of stromatoporoids Vertical (V) and diagonal (D) stromatoporoid parameters recorded at Long Quarry Point were exaggerated because of the combined effects of the dip of the strata (400 NNW) and the horizontal outcrop surface.

Figure 9. Combined effects of dip and horizontal outcrop surface on appearance of stromatoporoid dimensions, a) cross-section; b) semi-plan view. Surface A is normal to base of stromatoporoid and bears the true vertical (V) and diagonal(D) parameters. Surface B is an oblique section bearing the exaggerated observed parameters Vo and Do. Appendix gives method of correction.

Figure 9 illustrates the effects of tilting on the exposed surface of a domical stromatoporoid in growth position; the apparent vertical and diagonal dimensions (Vo and Do) are greater than true V and D. However, the basal parameter (B) is not affected because it represents the diameter of the base of stromatoporoids and is not, therefore, altered by dip. Assuming a dip of 400 (Fig. 9) and a horizontal outcrop surface, Vo and Do can be corrected as follows. Vo is corrected by: V= Cos 500Vo

V=U.64Vo This correction was also applied to the length of the transect and segments in order to obtain their true thicknesses shown in Figure L Do, with θ= 250 (Fig. 9b), is corrected by: D =[0.64+ (25 (1.00-0.64) ] Do

90 D = 0.74Do

These corrections permit good approximations to the real values of the parameters V and D to be made for specimens which are upright with respect to bedding, but it is difficult to apply corrections to forms which have been disorientated significantly. Nevertheless, in order to maintain uniformity of approach, the above corrections have been applied to all the stromatoporoids in the section.

The Lyme Regis (1901) Borehole succession and its relationship to the Triassic sequence of the east Devon coast G. WARRINGTON R.C. SCRIVENER

Introduction In a palynological study of the Permian (?) and Triassic deposits of the east Devon coast, productive samples were obtained only from beds some 135 to 185m above the base of the Mercia Mudstone Group and from the uppermost 8.75m of that unit (Stevenson and Warrington 1971; Warrington 1971). In consequence of these unsatisfactory results and because of the, then, inadequate documentation of the Mercia Mudstone Group in the coastal outcrops, accurately localised core samples from nearby boreholes were sought. Sources of such material are scarce and attention focused upon the Lyme Regis Borehole which was drilled by Vivian's Boring and Exploration Company, Limited, in 1901; the site (SY 336 930) was on the Lias outcrop some 5km north-east of the most easterly exposure of the Mercia Mudstone Group which is at Charton Bay, 5km east of Seaton in east Devon (Fig. 1). Core specimens representing 50 horizons in the borehole (Fig. 2) were located (largely by R.C.S.) and their lithologies were recorded. Suitable material was sampled for palynological study. The succession in the borehole has been reassessed in the light of this lithological examination, and on the basis of an analysis of published accounts (Jukes-Browne 1902; Richardson 1906; Woodward and Ussher 1906, 1911; Whitaker and Edwards 1926) and manuscript material, including the driller's record, held by the Institute of Geological Sciences. The results of this review of the lithostratigraphical sequence, summarised in Table I, are described below and depicted in Fig. 2.

The Lyme Regis Borehole The borehole, which was terminated at a depth of 396.85m, remains the only one in Devon and Dorset in which a substantial thickness of Triassic rocks has been cored. An account of the succession proved at Lyme Regis, and of its relationship to that seen at outcrop farther west, was published by Jukes-Browne in 1902. A summary log of the borehole was published by Woodward and Ussher (1906, 1911) and a slightly expanded version was made available by Whitaker and Edwards (1926). The stratigraphical interpretation by Jukes-Browne was slightly modified by Woodward and Ussher. Further amendment is made here (Table I and Fig. 2), particularly with regard to the interpretation, subdivision and nomenclature of the lithostratigraphical succession proved beneath the Lias. The nomenclature of that succession is here revised in accordance with the proposals formally introduced in the report of the Triassic Working Group of the Geological Society of London (Warrington and others 1980). The thickness of superficial deposits proved in the borehole is taken from the published accounts (see above) in which consistent values are cited for that unit. The separation of the Lias from the Langport Member (formerly the “White Lias”) of the Lilstock Formation is, on the basis of the available records, a subjective matter; the writers have placed that boundary at a depth of about 22.25m, as selected by Jukes-Browne (1902) and accepted by Woodward and Ussher (1906, 1911). The base of the Langport Member is placed at 28.98m and the unit rests

G. Warrington and R.C. Scrivener 1980. The Lyme Regis (1901) Borehole succession and its relationship to the Triassic sequence of the east Devon coast. Proc. Ussher Soc., 5, 24-32. The sub-Lias succession proved in the Lyme Regis Borehole in 1901 comprises a representative of the Penarth Group and a substantial, but incomplete, Mercia Mudstone Group sequence. Only one unit, l lm thick, in the predominantly argillaceous Mercia Mudstone Group is characterized by intercalated sandstones. The mudstones above that unit incorporate a significant amount of bedded gypsum; a mudstone unit beneath the sandstones is prominently colour-handed. Comparison of the succession drilled at Lyme Regis with that exposed on the east Devon coast suggests that the latter, though interrupted by faulting, comprises the full local Mercia Mudstone Group succession. As little as 33.5m of that unit may have remained unpenetrated below the terminal depth of the borehole. A recent interpretation of the exposed Mercia Mudstone Group sequence of east Devon as comprising three, mudstone units separated by two sandstone-bearing units is not supported either by field evidence or by comparison with the Lyme Regis Borehole succession. G. Warrington, Institute of Geological Sciences, Ring Road Halton, Leeds LS15 8TQ; R.C. Scrivener, Institute of Geological Sciences, St Just, 30 Pennsylvania Road, Exeter EX4 6BX

Figure l. Location map. Outline geology based, with permission, on Institute of Geological Sciences 1: 63360 Sheet 326/340, (Sidmouth and Lyme Regis).

upon a 1.88m-thick green or grey marl identified here as the Cotham Member of the Lilstock Formation (Fig. 2). That green or grey marl formed part of the units termed “shales of the Avicula-contorta zone” and “Black Shales” of the “Rhaetic Beds” by Jukes-Browne (1902, pp.286, 288) but, though noted by Woodward and Ussher and by Whitaker and Edwards as a unit separate from the underlying dark shales, it does not appear to have been distinguished as the “Cotham Beds”, the forerunner of the Cotham Member.

The base of the Cotham Member, at 30.86m, defines the base of the Lilstock Formation which overlies the Westbury Formation. The horizon adopted here (Table 1; Fig. 2) for the base of the latter unit corresponds with that assigned to the base of the “Black Shales” of the “Rhaetic Beds” by Jukes-Browne (1902, p.288) but lies within the “Rhaetic Beds” of Woodward arid Ussher (1906, p. 19) and above the “Passage Beds” subsequently distinguished by those authors (1911, p.20) as the lowest unit within the “Rhaetic Beds”. The base of the Westbury Formation also marks .the Penarth Group - Mercia Mudstone Group boundary (Table 1; Fig, 2).

The Blue Anchor Formation, the highest unit in the Mercia Mudstone Group, incorporates the “Grey Marls” recognised by Jukes-Browne (1902, p.288) and the highest (green) beds of his underlying “Marls, without gypsum” (Jukes-Browne 1902, pp.284-5, 288). The formation is equivalent to the lowest unit distinguished within the “Rhaetic Beds” by Woodward and Ussher (1906, 1911) in addition to the underlying “Green marl” assigned by them to the “Keuper Marl”. As such, it is broadly equivalent to, the combined “Grey Marl” and “Tea Green Marl” units of former nomenclature.

The graphic representation of the Mercia Mudstone Group succession beneath the Blue Anchor Formation (Fig. 2) corresponds broadly with earlier interpretations but renders apparent features that Were less evident in previous accounts. These features may prove relevant for a formal lithostratigraphical subdivision of those deposits and are as follows:

a. Mudstones between 65.48m (the base of the Blue Anchor Formation) and 90.63m apparently lack sulphate minerals. b. Sulphate minerals appear at a depth of 90.63m and are present, in varying amounts, throughout the underlying strata as veins, beds and nodular developments. Beds of sulphates were recorded between 130.05m (driller's log) and 277.14m (driller's log, and Jukes-Browne, 1902); the positions and thicknesses of these beds, taken from the driller's log, are indicated alongside the graphic section (Fig. 2). A concentration of beds up to 0,41m thick, and with a combined thickness of 3.61m, was recorded between 165.58m and 180.04m. More widely spaced beds of sulphates over 0.2m thick occurred as high in the borehole as 158,12m and as low as 197.00m (Fig. 2).

c. The succession between 252.35 and 263.35m comprised gypsiferous mudstones with four beds of “Grey marly limestone” and “Grey limestone with some marl” (driller's record) or of “Grey limestone” identified, in the two instances in which specimens were seen, as “calcareous sandstone” (Jukes-Browne 1902, p.285). In a summary of the Lyme Regis succession, Jukes-Browne (1902, p.288), mentioned only three beds of calcareous sandstone but four such units are specified in his more detailed log (Jukes-Browne 1902, p.285) which corresponds, in this respect, with the driller's record and was evidently the basis for Whitaker and Edwards' summary. The presence of four beds of calcareous sandstone in the 11m-thick unit concerned is accepted here (Fig. 2). d. The mudstones between 292.20m and 323.62m consist mainly of brown and blue “shale” with gypsum (driller's record) and appear to be prominently colour-banded. Beds of “blue shale” which are mentioned specifically in the driller's record, are restricted to that depth interval and-have a combined thickness of 7.57m, the thickest bed being 3.20m. e. From 365.76m to the terminal depth, specimens noted in Jukes-Browne's detailed account (1902, p.286) were described as silty though in a summary log (Jukes-Browne 1902, p.288) the top of the silty unit was placed at 354.05m. The latter division was also adopted by Woodward and Ussher (1906, 1911). In this account, the presence of silty lithologies at and below 365.76m is recognised (Fig. 2). Comparison and correlation with the east Devon coastal outcrop succession In the absence of specimens or records of ammonites from the Lias of the borehole section comparisons are restricted to the sub-Lias sequence and are made on a strictly lithostratigraphical basis. Table 1: the revised lithostratigraphical succession from the Lyme Regis Borehole Thickness To depth of Superficial deposits 3.25m 3.25m Lias c. 19.00m c. 22.25m Penarth Group (i) Lilstock Formation c. 8.61 m 30.86m

(Langport Member) (c. 6.73m) (28.98m) (Cotham Member) (1.88m) (30.86m)

(ii) Westbury Formation 9.88m 40.74m Mercia Mudstone Group (i) Blue Anchor Formation 24.74 m 65.48m (ii) undifferentiated 331.74m seen 396.85m

mudstones (Terminal depth)

Figure 2. Graphic interpre-tation of the succession proved in the Lyme Regis Borehole with, for comparison, an earlier summary (after Jukes-Browne 1902). and the horizons of specimens in the collections of the Torquay Natural History Society, the Exeter and Tauntcn museums and the Institute of Geoloeical Sciences.

Figure 3. (Top) The south Devon coastal section from Higher Dunscombe Cliff eastwards to Weston Cliff (from photographs by G. Warrington and A.J.J. Goode). (Bottom) Interpretation of the above photographs showing major lithostratigraphical units, areas obscured by vegetation (black), landslips, and topographical features.

Figure 4. Oblique aerial view of Lower Dunscombe Cliff from the east. Eastern end of Weston Cliff and the gully at Weston Mouth are in the foreground. The Lower Dunscombe Cliff landslip and Kempstone Rocks are in the middle distance and Higher Dunscombe Cliff is in the background. (Photograph by courtesy of the Commanding Officer, 3rd Commando Brigade Air Squadron, Plymouth).

those elsewhere in southern and central England. Significant arenaceous units (e.g. the North Curry Sandstone and the Arden Sandstone members of Somerset and the central Midlands respectively; see Warrington and others 1980) occur at only one level in the group in those areas. The arenaceous unit present between 252.35m and 263.35m in the borehole lies 211.61 m below the Penarth Group. and 72.31m below the concentration of beds of sulphates. The thickness for the corresponding strata which outcrop between the base of the sulphate-rich unit at Branscombe Ebb and the top of the arenaceous unit in Weston Cliff is, from Jeans' account, a maximum of 90m. As that figure includes estimates covering two gaps in the exposed succession, the actual value may be rather less and possibly not significantly different from the corresponding one computed from the borehole section some 17.5km to the east-north-east (Fig. 1). The outcrop succession equivalent to that present below the arenaceous unit recognised in the borehole comprises the strata exposed beneath the arenaceous beds present in Weston Cliff and the Dunscombe cliffs and which are here regarded as comprising a single unit, the Weston Mouth Sandstone Member. That succession is almost continuously exposed in Higher Dunscombe Cliff and, westwards towards Sidmouth, in Maynard's and Salcombe Hill cliffs (Fig. l). The group rests upon the Otter Sandstone Formation of the Sherwood Sandstone Group at Sidmouth. Prominently colour-banded mudstones were encountered in the borehole between 292.20 and 323.62m in a unit extending from 28.85 to 60.27m below the arenaceous beds. A comparable colour-banded unit is prominently displayed in Higher Dunscombe Cliff where, from Jeans' account, it occurs from 36.58 to 59.72m below the arenaceous unit visible higher in that cliff. Allowing for the distance of about 19km between that outcrop and the borehole site, these values compare tolerably well with those expressed above for the corresponding unit in the latter section. In the coastal outcrop, Jeans (1978) recorded some 167m of the Mercia Mudstone Group between the arenaceous unit in Higher Dunscombe Cliff and the base of the group at Sidmouth. On the basis that the arenaceous beds in the Dunscombe and Weston cliff sections comprise a single unit and correlate with that recognised in the borehole, below which 133.50m of the Mercia Mudstone Group were proved (Fig. 2), a difference of only 33.5m is seen to exist between the known thickness of those mudstones at outcrop and the incomplete thickness of the corresponding beds proved in the borehole. The base of the group at Lyme Regis may, therefore, be as little as 33.5m below the depth at which the borehole terminated (i.e., at about 430.35m) or, if the sequence thickens gradually eastwards

into the Wessex Basin, at only slightly greater depth. The minimum thickness of the Mercia Mudstone Group at Lyme Regis is thus estimated to be about 390m. The sections of the Mercia Mudstone Group seen at outcrop between Culverhole and Sidmouth are interrupted by faults. However, from the correspondence between the borehole and outcrop sequences, demonstrated above, it is implicit that the full succession of the group is exposed in the coastal outcrops in east Devon. Conclusions 1. The Mercia Mudstone Group of the Lyme Regis Borehole section includes a major concentration of sulphates at 165.58 to 180.04m and one significant sandstone-bearing unit at 252.35 to 263.35m. 2. Close lithostratigraphical similarity is demonstrable between the Mercia Mudstone Group successions of the Lyme Regis Borehole and the east Devon coastal outcrops if the latter are regarded as including only one significant arenaceous unit. 3. The coastal outcrops between Sidmouth and Culverhole, east of Seaton, display the full Mercia Mudstone Group sequence present in east Devon. 4. The Lyme Regis Borehole may have terminated as little as 33.50m above the base of the Mercia Mudstone Group which has an estimated minimum thickness of some 390m at that site. The estimated minimum depth of the base of the group there is 430.35m. Acknowledgements. The writers are grateful to the Torquay Natural History Society and the authorities at the Exeter and Taunton museums for the facilities granted for the examination of Lyme Regis Borehole specimens held in their collections, and for permission to sample suitable specimens for palynological study, the results of which will be published separately. The constructive comments made by Mr R.S. Arthurton, Mr G. Bisson and Dr A. Whittaker on an initial version of this account are acknowledged with appreciation. The aerial photograph of Lower Dunscombe Cliff(Fig. 4), reproduced here by courtesy of the Commanding Officer, 3rd Commando Brigade Air Squadron, Plymouth, was kindly drawn to our attention by Dr D.J.C. Laming. References Jeans, C.V. 1978. The origin of the Triassic clay assemblages of

Europe with special reference to the Keuper Marl and Rhaetic of parts of England. Phil. Trans. R. Sec., A.289, 549-639.

Jukes-Browne, A.J. 1902. On a deep boring at Lyme Regis. Q. Jl geol. Soc., Lond., 58, 279-289.

Richardson, L. 1906. On the Rhaetic and contiguous deposits of Devon and Dorset. Proc. Geol. Ass., 19, 401-409.

Stevenson, C.R. and Warrington, G. 1971. Jurassic and Cretaceous rocks of Wessex: Highest Keuper Deposits. Written discussion to report of field meeting. Proc. Geol. Ass., 82, 297-300.

The Penarth Group The thicknesses assigned to the Lilstock Formation and its component members in the Lyme Regis Borehole section (Table 1) compare tolerably well with those cited by Richardson (1906) for the corresponding units at outcrop at Charton Bay and Culverhole, respectively 5 and 7km south-west of the borehole site (Fig. 1). At outcrop, the Langport Member (“White Lias”) was 7.62m thick (estimated) and the Cotham Member (“Upper Rhaetic”) was 1.75m (Richardson 1906); in the borehole those units were c, 6.73m and 1.88m thick respectively.

A significant difference occurs between the thickness assigned to the Westbury Formation (“Lower Rhaetic”) at outcrop (5.13m; Richardson 1906) and that recognised in the borehole section (9.88m; Table 1). The former is regarded as a considerable under-estimate resulting from the exposures of the formation at Culverhole being subject to landslip. The thickness of the unit at outcrop is likely to be similar to that observed in the borehole and the estimate of 9.14m made by Jukes-Browne (1902, p.281) for the outcrop section is probably very close to reality.

The Mercia Mudstone Group The Blue Anchor Formation (“Tea-Green and Grey Marls”) is 23.01m thick at outcrop at Culverhole (Richardson 1906, p.402) and 24.74m in the borehole, a tolerable correspondence over a distance of about 7km (Fig. I). The Mercia Mudstone Group succession present beneath that formation, and which is exposed between Haven Cliff, Seaton, and Sidmouth (Fig. 1), was largely undocumented until the publication of an account of the clay mineralogy of the “Keuper Marl” (Jeans 1978) in which detailed lithostratigraphical documentation of the coastal sections was presented.

A massive nodular sulphate-bearing unit, 6 to 7.5m thick, crops out in the Mercia Mudstone Group at Branscombe Ebb (Fig. 1) and is regarded by Jeans (1978, p.634) as a “very reduced lateral equivalent” of the “Clays, with beds of gypsum” recognised by Jukes-Browne (1902, p.288) in the Lyme Regis Borehole, This correlation is improbable, for the unit of which Jeans considers the outcropping sulphate-rich beds to be a thin lateral equivalent is 95.66m thick in the borehole (Jukes-Browne 1902). A thinning of that magnitude is unlikely, and the Branscombe Ebb sulphate-rich unit is here considered to be the equivalent, at outcrop, of at least the upper part of the concentration of beds of sulphates shown (Fig. 2) to occur from 124.84 to 139.30m below the Penarth Group in the borehole. By comparison with the latter section, the base of the exposed sulphate-rich unit is considered to occur some 50m higher in the sequence than proposed by Jeans who (1978, p.632) regarded it as being 181m below the Penarth Group. The thickness of Mercia Mudstone Group rocks, comprising the sulphate-rich beds and overlying strata, exposed in Berry and Branscombe West

cliffs (Fig. 1), is some 58m (Jeans 1978, p.559). If the sulphate-rich beds at Branscombe Ebb are correlated with the higher part of the concentration of similar beds in the borehole, it is evident that about 73m of the Mercia Mudstone Group sequence above that concentration are unrepresented in the coastal outcrops immediately to the east of Branscombe Ebb.

The Mercia Mudstone Group is concealed beneath Cretaceous rocks to the east of Branscombe Mouth (i.e. towards Beer Head) but reappears, beyond a major fault, farther east around Seaton (Fig. 1). Some 64m of strata, including the Blue Anchor Formation, occur in cliffs to the east of Seaton (Jeans 1978). If this thickness is subtracted from that of about 73m calculated above for the amount of the higher Mercia Mudstone Group unrepresented between Branscombe Mouth and Beer Head, it appears that a stratigraphical interval of as little as 9m may exist between the highest unit in the group seen in the Branscombe cliffs and the lowest seen in those to the east of Seaton. This gap could be covered by the sequence outcropping westwards from Seaton to Seaton Hole, a section not documented by Jeans. In consequence, it is concluded that strata equivalent to the Mercia Mudstone Group succession present between depths of 40.74m and about 130m in the borehole (i.e. between the top of the group and a level regarded as correlating with the base of the exposed sulphate-rich beds) are fully represented in the outcrops between Branscombe Ebb and Culverhole.

Jeans (1978) has proposed that two distinct arenaceous units, designated as “sandstone groups” within the “Dunscombe' and “Weston” cycles, occur in the exposed Mercia Mudstone Group beneath the horizon of the Branscombe Ebb sulphate-rich unit. The “Dunscombe” cycle, visible in Higher and Lower Dunscombe cliffs (Fig. 1) is regarded by Jeans as older than and at least 17m lower in the succession than the “Weston” cycle recognised to the east in the adjacent Weston Cliff (Fig. 1). On field evidence alone this interpretation is unacceptable. The continuity of outcrop of an arenaceous unit seen in situ in Higher Dunscombe and Weston cliffs, is disrupted by a major landslip in the intervening Lower Dunscombe Cliff area (Figs 3, 4). The arenaceous unit is not in situ in that cliff, as suggested by Jeans (1978, p.635) but is displaced by and incorporated wholly within that landslip. Jeans' interpretation of the arenaceous units of the Dunscombe and Weston cliff sections as stratigraphically separate entities is not, therefore, adopted in the following comparison of the lower part of the Mercia Mudstone Group of the Lyme Regis Borehole with that at outcrop. Close litho-stratigraphical similarity is demonstrable between those sequences if only one arenaceous unit, the Weston Mouth Sandstone Member (Warrington and others 1980). is regarded as present at outcrop. If this interpretation is followed, similarity also exists between the Mercia Mudstone Group succession of the Devon coast and

Warrington, G. 1971. Palynology of the New Red Sandstone sequence of the south Devon coast. Proc. Ussher Soc., 2, 307-314.

Warrington, G,, Audley-Charles, M.G., Elliott, R.E., Evans, W.B., Ivimey-Cook, H.C., Kent, P.E., Robinson, Pamela L., Shotton, F.W. and Taylor, F.M. 1980. A correlation of Triassic rocks in the British Isles. Spec. Rept Geol. Soc. Lond., No. 13, 78pp.

Whitaker, W. and Edwards, W. 1926. Wells and Springs of Dorset. Mem. geol. Surv. U.K., xi + ll9pp.

Woodward, H.B. and Ussher, W.A.E. 1906. The geology of the country near Sidmouth and Lyme Regis. Mem. geol. Surv. U.K., vi + 96pp.

Woodward, H.B. and Ussher, W.A.E. 1911. The geology of the country near Sidmouth and Lyme Regis. (Second edition). Mem. geol. Surv. U.K., vi + 102pp.

This contribution is published by permission of the Director, Institute of Geological Sciences.

Superposed folding at Rosemullion Head, South Cornwall W.R. DEARMAN B.E. LEVERIDGE R.P. RATTEY D.J. SANDERSON

W.R. Dearman, B.E. Leveridge, R.P. Rattey and D.J. Sanderson 1980. Superposed folding at Rosemullion Head, South Cornwall Proc.. Ussher Soc.. 5, 33-38. An example of supposed 'crossfolding' with two sets of co-planar fold axes in the Gramscatho Beds of South Cornwall is examined. An analysis of the fold axis distribution and bedding orientations indicates that two phases of folding are present and examples of the interference of these are described. W.R. Dearman, Department of Geology, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne NEI 7R U; B.E. Leveridge, Institute of Geological Sciences, St Just, 30 Pennsylvania Road, Exeter EX4 6BX; R.P. Rattey and D.J. Sanderson, Department of Geology, Queen's University of Belfast, Belfast BT7 INN

Introduction Rosemullion Head is situated on the south coast of Cornwall, some 5km south of Falmouth (Fig. 1). Gramscatho Beds (Hendriks 1937) are well exposed in wave-cut platforms around the low drift-capped headland. They are a Middle Devonian flysch facies sequence of alternating greywacke sandstones and mudstones. Sandstones, up to 2m thick, are graded and commonly show complete Bouma sequences with flutes, grooves and load casts on the bases of the units. The pattern of structures in the Gramscatho Beds was described by Lambert (1959) who recognised one major phase of deformation. The folds were categorised, on the basis of the trend of their fold axes, as 'main' folds, trending ENE-WSW, and 'cross' folds, trending NW-SE, both sets having a common axial Planar cleavage. This work has been quoted (e.g., Whitten 1966) as an example of synchronous single-phase crossfolding although, in a later paper, Lambert (1966) suggested that the two fold trends could be developed on different limbs of earlier open folds, which were not observed. Observations by Turner (1968) on the Gramscatho Beds of the Gunwalloe-Porthleven area and contemporaneous work by one of the authors (BEL) on the Gramscatho Beds of Roseland showed the early tectonic fabric in the pelites to be a slaty cleavage (S1) A well developed second cleavage (S2), that locally transposes S1, is a crenulation cleavage in pelites and a spaced cleavage in psammites. These two phases of deformation have now been recognised throughout south-west Cornwall (Sanderson 1973; Rattey 1980). The interference of the two distinct phases of folding is well exposed on Rosemullion Head and is described in this paper.

First phase folds (F1) F1 folds are present throughout the section. They are close to tight folds with axes plunging to the N or NE and S or SW (Fig. 2). On the long limbs of F2 folds, the F1 folds are gently to moderately inclined. The long limbs are right-way-up and the short limbs inverted and fold pairs verge to the NW or W. Sedimentary structures indicate that the F1 folds also face to the NW or W. The attitude of the F1 folds and S1 cleavage is modified by subsequent folding (F2) which steepens and overturns the earlier structures.

Second phase folds (F2) F2 folds are the most conspicuous minor folds of the area and clearly deform an earlier fabric (S1). The folds are open to close, with rounded hinge zones, and have an axial planar cleavage (S2) which is a crenulation cleavage in pelites and a spaced cleavage in psammites. S2 dips gently or moderately SE and, in F2 hinge zones, divergent fans in pelites and convergent fans in psammites are common. Locally S1 is transposed by S2, the latter appearing penetrative in hand specimen. The larger mesoscopic F2 folds have long gently dipping limbs and shorter steeply inclined limbs and are overturned or verge northwards. Two fold plunge directions, ENE and ESE, predominate (Fig. 2) with some variation in orientation.

Interference of F1 and F2 Structures produced by the two deformation phases are ubiquitously developed and good examples of their interference are common.

An F1 fold at locality 1 (Fig. 1) is developed on the long lower limb of an F2 fold. At locality 2, in the hinge region of the F2 fold, small F2 folds clearly refold S1 and the S1/Ss intersection At locality 3, an F1 fold hinge is exposed in the steep limb of a large F2 fold. The F1 hinge has a steep, variable plunge due to refolding by F2. On the southern side of Rosemullion Head (locality 4)an F1 fold is exposed on the wave cut platform. S2 cleavage, dipping gently SE, crosses from the normal to inverted limb of an F, fold. Minor F2 folds are present on both limbs of the F1 fold, which on the long normal limb have axes trending ENE and facing NNW, whereas on the short inverted limb the axes trend ESE and face SSW. A general feature of both the F1 and F2 folds is their similarity in size., with short limbs commonly about 20m in length, and the scarcity of smaller folds with easily observable interference patterns. It is possible to delimit structural domains (or sub-areas), within which the various mesofabric elements (Ss, S1, S2 and fold axes β1 and β2) maintain a fairly constant orientation; these generally occupy 20m to 100m of coastal exposure. In general, n phases of folding will produce 2n domains, and four domains are recognised in the Rosemullion area, produced by the two phases of folding. Table 1 summarises the orientations of the mesofabric elements in these domains and formalises the previous descriptions

of the structures. In other areas away from Rosemullion Head the attitudes of both the F1 and F2 folds vary. Hence the orientation of the fabric elements in Table 1 strictly applies only to this area although the general pattern can be recognised more widely. The evolution of these domains can be considered. Prior to F1 folding, bedding (Ss) had a constant attitude and one domain existed (n = 0). F1 folding produced two differing attitudes of Ss and hence two domains (n = 1). These are domains 1 and 3 in Table 1 and their present positions are shown in Fig. 3. F2 folding affected both these domains. On the normal limbs of F1 folds, folding about β2 a gave rise to domains 1 and 2 (Fig. 3a). On the inverted limbs folding about β2b produced domains 3 and 4. β2a and β2b have differing attitudes (significantly different at < 0.001 level assuming a Fisherian distribution), but, since they fold the normal and inverted limbs, respectively, of earlier tight F1 folds, they are further distinguished by opposing facing directions within S2 (Table 1). Clear, mappable interference patterns involving all four domains can be recognised at locality 5 (Fig.1). Elsewhere the domains rarely map out over many folds due to minor faulting. By combining several observed field relationships and using the modal orientations from the domains discussed above a diagrammatic reconstruction of the interference of the F1 and F2 folds has been made (Fig. 4).

Table 1. Orientation of fabric elements in the structural domains at Rosemullion Head. The data represent modal values and there is some variability within domains.

DOMAIN 1 2 3 4

Bedding (Ss) c.30°NNE c.65°SSE c.45°ENE c.50°SSW younging normal inverted inverted normal

F1 facing W WNW W WNW vergence W ESE E WNW

β1 plunge gently N mod. SSW gently N mod. SSW S1 dip mod. NE steeply S mod. NE steeply S

F2facing up N up N down SSW down SSW vergence N S NNE SSW

β2 plunge Gently E or ENE Gently/ Mod. ESE S2 dip Gently/ moderately to SE

Domain represents: F1 long limb long limb short limb short limb F2 long limb short limb long limb short limb

Figure 2. Equal-area stereograms of fold axis distribution (bars on plotted points indicate facing directions). a) Data from Rosemullion Head. b) Data for area around Helford River, including Rosemullion Head (after Lambert 1959).

Figure 3. Equal-area stereoarams of poles to bedding (dots) and F2 fold axes (circles). For further explanation see text.

Figure 4. Diagrammatic representation of refolding of F1 folds by F2, Rosemullion Head. Shaded surface shows stratigraphical top of hypothetical layer folded about β1, β2a. and β2b axes. Numbers in circles indicate position of the 4 domains in Table 1.

Discussion Lambert (1959) described the fold axis distribution as due to a single phase of folding producing 'main' and 'cross' folds, which both share a common axial plane. Whilst such distributions are known to exist in oblique fold systems (Sanderson 1973) and such systems are locally developed in various parts of Cornwall (Dearman 1969; Sanderson 1972, 1973), a study of the folding at Rosemullion Head shows that two distinct fold phases are developed. Lambert (1966) later accounted for the two groups of fold axes by inferring an earlier phase of open folding (his Fo), with 'main' and 'cross' folds developed by later folding of different limbs of these Fo folds. This explanation is similar to our observations and analysis of the development of the two sets of F2 fold axes, except that we have now observed the earlier folds (our F1) as close to tight overturned folds which thus produce opposing facing directions of the later F2 folds. It should be noted (Fig. 2b) that Lambert's 'cross-folds' include both F1 and F2b folds and the 'main' folds consist mainly of F2a folds but may include F1 folds on the long limbs of the major F1 folds (compare Fig. 2a and 2b). Summary The pattern of deformation within the Gramscatho Beds of Rosemullion Head, South Cornwall, previously ascribed to synchronous crossfolding during one main phase, has been reinterpreted. Two major deformations are recognised, the early phase (F1) characterised by close to tight folds associated with slaty cleavage and a second phase (F2) of open to close folds and crenulation cleavage. The well-defined trend domains of F2 folds that gave rise to the 'crossfolding' concept of Lambert (1959) are seen to be the product of the interference of structures of the two phases. Each limb of an F1 fold forms a separate F2 trend domain. The trend and facing of the F2 folds depends upon which of the divergent limbs of an F2 fold they are developed. There is considerable regional variation in the intensity and orientation of F1 and F2 structures and hence in their interference. In restricting this discussion to one small area we seek to establish clearly the polyphase nature of the folding. Further discussion of the regional variation will be presented elsewhere (see Rattey 1980). References Dearman, W.R. 1969. Tergiversate folds from south-west

England. Proc. Ussher Soc., 2, 112-115. Hendriks, E.M.L. 1937. Rock succession and structure in South

Cornwall, a revision. With notes on the Central European facies and Variscan folding there present. Q, Jl. geol. Soc. Lond., 93, 322-360.

Lambert, J.L.M. 1959. Cross folding in the Gramscatho Beds at Helford River Cornwall. Geol. Mag., 96, 489496.

Lambert, J.L.M. 1966. The structure of south-west Cornwall: a study of tectonic facies. Proc. Ussher Soc., 1,218-220.

Rattey, P. 1980. Deformation in S.W. Cornwall. Proc. Ussher Soc., 5, 39:43.

Sanderson, D.J. 1972. Oblique folds in south-west England. Proc. Ussher Soc., 2, 438-441.

Sanderson, D.J. 1973. The development of folds oblique to the regional trend. Tectonophysics, 16, 55-70.

Sanderson, D.J. 1973. Correlation of fold phases in S.W. England. Proc. Ussher Soc., 2, 525-528.

Turner, R.G. 1968. The influence of granite emplacement on structures in south-west England. Ph.D. thesis, University of Newcastle-upon- Tyne.

Whitten, E.H.T. 1966. Structural Geology of Folded Rocks. Rand McNally & Co., Chicago, 663pp.

Read at the Annual Conference of the Ussher Society, January 1980

Deformation in south-west Cornwall R.P. RATTEY

R.P. Rattey 1980. Deformation in south-west Cornwall. Proc. Ussher Soc., 5, 39-43. The general deformation history within the Mylor and Gramscatho Beds of south-west Cornwall is described with particular emphasis on the regional structural variation. Five phases of deformation have been identified. R.P. Rattey, Department of Geology, Queen's University of Belfast, Belfast B77 INN

Introduction It has long been recognised that the Mylor and Gramscatho Beds cropping out in the area outlined in Fig. 1a have been intensely deformed during the Variscan Orogeny. De la Beche (1839) described these rocks as highly contorted, but Hendriks (1937) was the first to attempt a regional structural synthesis. Her concept of a major nappe structure produced by the emplacement of the Lizard complex revolutionised the structural inter-pretation of south Cornwall and was adopted by Flett (1946) for the new memoir and for the 1:63360 map (1949). Since Hendrik's initial observations much of the structural research has concentrated on relating the intense folding and cleavage development within the Meneage olistostromes and Gramscatho Beds to the emplacement of the Lizard complex, This is typified by the approach of Hendriks (1959; 1970) and Hendriks and others (1971) where large scale structures are described without reference to deformation chronologies or variation in both the attitude and the spatial distribution of minor structures. More recent studies have involved the construction of detailed deformation chronologies. The work of Lambert (1959), Smith (1965), Stone (1962, 1966), Turner (1969) and more recently Dearman and others (1980) and Hobson (1977) permits the correlation of deformation phases over large areas (Sanderson 1973b) and hence a description of the regional structural variation (Sanderson and Dearman 1973). The relationships between the structural variation and major geological events such as granite or ophiolite emplacement may then be discussed. This is the approach adopted in this study.

First deformation (D1) Fl folds are tight and are sub-horizontal or plunge gently ENE or WSW. They verge and face to the NNW and have axial planes which dip gently SSE. The Fl fold envelope dips gently SSE, but is modified by later deformation. Associated with the folding is an intense slaty cleavage (S1) which lies sub-parallel to the Fl long limbs. The S1/ bedding (Ss) intersection lineation is well defined on Fl short limbs and lies parallel to the fold axes (�1). Fl fold style varies with lithology. Within the Mylor Beds Fl folds are tight to isoclinal and have sub-rounded hinges with trend towards class 2 types (Ramsay 1967). In the greywackes of the Gramscatho Beds they are typically tight, flattened chevron folds approximating class lc types. They are commonly on a larger scale than within the Mylor Beds due to the chevron folding of thicker bedding units (see Sanderson 1974 and Ramsay 1974). Accommodation structures are frequently developed in response to the folding in both lithologies and involve the local thrusting and flexing of fold limbs. No major inversions of the stratigraphy by overfolding in response to thrusting, as suggested by Hendriks (1970), have been found, but small areas of SSE verging and NNW facing F1 folds have been recorded from Jangye Ryn (659 207), Blackcliffs (555 390), and Gillian Creek (788 254). Mapping at these localities has revealed first order F1 folds with short limbs up to 200m in length. The SSE verging minor F1 folds are developed on the short limbs of these larger folds. S1 is an intense slaty cleavage in the pelites of the Mylor Beds, but is a spaced cleavage in the psammites of the Gramscatho Beds. It is poorly developed in the basal, coarsely graded layers (Unit A of Walker 1967), but increases in intensity upwards into the

shale-rich upper units. A weak stretching lineation is infrequently developed and plunges gently SE; however, pressure shadow fringes about pyrite cubes indicate that the finite D1 strain has been dominantly oblate. In thin section chlorite porphyroblasts up to 0.2nlm in length lie parallel to S1 and the segregation of quartz into veins is frequently observed. Variation in D1 Structure Two major zones of reoriented F1 folds have been noted. A zone of recumbent F1 folds plunging gently ENE or WSW and facing and verging NNW has been recorded from the N coast section between Godrevy Point (579 434) and Porthtowan (690 480). Their recumbent attitude is similar to the F1 folds exposed to the NE at Newquay (Ripley 1965), Perranporth (Sanderson 1971; Henley 1973), and Padstow (Gauss 1973; Roberts and Sanderson 1971). This change from recumbent to inclined folds forms the basis of the transition from Zones 9 to 10 of Sanderson and Dearman (1973). The rotation of F1 axial planes is interpreted as due to coaxial refolding about a major, upright, gentle, F2 antiform (Fig. la) which crops out in the complex Godrevy Point to Blackcliffs section and has been termed the Godrevy Antiform (Fig. lb). North of the Lizard complex, Dearman (1969) recognised zones of Fl folds with axes trending N-S and hence oblique to the regional trend. These folds are open to close flattened chevron folds which face and verge to the W. D2 deformation is superposed on these Fl folds and the geometrical interference Patterns produced indicate that the oblique attitude of Fl folds developed prior to De and hence during the D1 deformation phase. The attitudes of Fl folds on the long limbs of F2 folds at Rosemullion Head (Dearman and others 1980) are primary oblique Fl folds. The generation of these folds is not by simple rotation of fold axes towards a stretching lineation, as described by Sanderson (1972) for the high strain zone at Tintagel. The folds appear to have initiated in an oblique attitude and their generation will be discussed more fully elsewhere. Second deformation (D2) A second phase of deformation (D2) is superposed on Fl folds and crenulates S1 cleavage. It produces open to close folds which plunge gently ENE or WSW and face and verge NNW on the long limbs of Fl folds. Good examples of the regional Fl /F2 coaxial interference are seen at Godrevy Cove (579 430) and have been described by Smith (1965). F2 folds also occur in areas of primary oblique Fl folding and Dearman and others (1980) have described the non-coaxial superposition in detail. Generally the F2 fold envelope dips gently SSE, but is frequently modified by the intense D3 deformation. S2 cleavage is a moderately to steeply SSE dipping crenulation cleavage often associated with intense pressure solution striping. It lies axial planar to the F2

folds end the intersection of S2 and bedding is clearly defined on both fold limbs and is parallel to the fold axes (β2 ). F2 hinges are rounded and approximate to class 1c types with few accommodation structures developed. Fanning of cleavage through the fold hinges is common and refraction through differing lithologies prominent. D2 Shear Zones Variation in the style and intensity of D2 deformation occurs about the Godrevy Antiform. On the SE limb of the Godrevy F2 fold, zones of relative increase in D2 deformation have been observed in which S2 cleavage dips more gently SSE and the F2 folding intensifies. The zones have sharp margins and increase in number and intensity to the SE. Variation in the attitude of D2 shear zones and their resultant interference patterns with regional and oblique F1 folds has been noted. D2 shear zones shallow to the SE (Fig, 2) and the interference patterns vary from refolding under bed parallel shortening, where the zones dip more steeply than bedding, to rotation of F1 fold axes without refolding under bed parallel extension, where the zones dip more gently SE. Compressional crenulating fabrics are produced where D2 shear zones dip steeply, while composite extensional fabrics with associated stretching lineations are developed where the shear zones are gently inclined. The non-coaxial refolding of primary oblique Fl folds at Rosemullion Head, described by Dearman and others (1980) is the pattern produced by their interference with a moderately inclined D: shear zone. To the S at Gillan Greek a gently dipping D2 shear zone has been noted and F1 folds undergo finite extension with rotation of fold axes towards an intense stretching lineation on the resultant composite S1/S2 cleavage. Other zones where extension dominates and little refolding occurs have been observed in the Helford River (770265) and Poldhu (664 199) to Polurrian Cove (668 188) coastal sections. The attitude of stretching, which plunges gently SE, relative to the F1 fold axes produces opposing rotations for primary oblique and regionally orientated F1 folds and facing and vergence confrontations are locally developed. Third deformation (D3) F3 folds are the intensely developed recumbent minor folds associated with a fiat-lying axial planar crenulation cleavage that are well exposed along Porthleven Sands (635 250). Stone (1966) assigned these folds to a second phase of folding, but Smith (1965) and Turner (1969) both recognised two earlier phases and assigned them to the third deformation event. Turner (1969) suggested a relationship between the intrusion of the granite batholith and D3 deformation. This was confirmed by a study (Rattey 1979) of the Mount's Bay section across the Tregonning granite cupola.

Figure 2. Schematic NNW-SSE section showing how the superposition of variably oriented D2 shear zones produces variation in the observed interference patterns with bedding. D2 shear zones shallow to the SSE and interference changes from finite compressional in the NNW to finite extensional in the SSE, towards the Lizard Complex/ killas contact.

The D3 formation phase is complex and involves the early generation of a series of culminating synforms and antiforms above the roof of the granite batholith (Fig. lc). The minor F3 folds described by Smith (1965), Turner (1969), and Rattey (1979), together with those assigned F2 by Stone (1966) are superposed over these broad hinged, open culminating folds. Minor F3 folds verge away from the open D3 antiforms and towards the synforms, therefore the relationship is not one of two orders of folding, but a complex superposition of two related fold types. The observed two generations of folding in 93 agrees with the deformation models produced for the roof zone of a granite batholith emplaced by diapirism (Dixon 1975). Superposition of S3 cleavage, which strikes parallel to all the granite/killas contacts and the gravity anomaly isogals (Bott and others !958), over the early open folds produces a complex set of interference patterns. Along Porthleven Sands minor F3 folds on the long limbs of F1 · and F2 folds plunge gently ENE or WSW and face and verge SSE. This pattern, however, varies throughout SW Cornwall due to superposition over the variable D1 and D2 terrain. Individual granite cupolas all cross-cut the S3 cleavage, but the injection of products of late stage magmatic differentiation frequently lie parallel to S3. The post-magmatic metasomatic differentiation, producing banding (Stone 1969) also lies parallel to S3. To the SE of the Carnmenellis and Tregonning-Godolphin granites S3 cleavage steepens and rapidly decreases in intensity. No S3 cleavage has been observed to the SE of a line from Gunwalloe (653225), through the Helford River to Rosemullion Head. This corresponds to the southern steep wall of the granite batholith. Granite emplacement was complete prior to D4.

Late stage deformation (D4, Ds) Two phases of deformation post date D3. D4 deformation produced a NNW-SSE sub-vertical crenulation cleavage axial planar to open F4 folds which plunge moderately SSE. F4 folds are frequently developed in zones of intense D4 deformation as noted between Godrevy Point and Strap Rocks (579416) on the N coast and between Falmouth (805 325) and Maenporth (790 297). Outside the zones S4 cleavage and F4 folds are weakly and sporadically developed. S4 cleavage shows variability in orientation which is unrelated to the variation in the earlier fold phases. Between Falmouth and Maenporth S4 strikes NNE-SSW and dips sub-vertically. It is non-axial planar to the F4 folds which plunge moderately S and the S4/bedding intersection lineation is not parallel to the F4 fold axes. D5 deformation is weakly developed and involves both the generation of conjugate E-W trending sets of kink bands and rare broad, open E-W trending upright folds with associated steeply dipping axial planar crenulation cleavage. S5 cleavage can be observed crenulating S4 in the Godrevy Point section and has been described by Smith (1965). Conclusion The deformation history outlined is at variance with that described by Barnes and others (1979)and Lambert (1959). Both conclude that only one major phase of deformation pertained. Lambert (1959) explained the variation in attitudes of fold axes as due to crossfolding, but Dearman and Others (1980) have shown that the resultant interference patterns are due to refolding by a

distinct D2 deformation. The deformation history described confirms and extends the work of Smith (1965), Turner (1969), and Sanderson (1973). Acknowledgements. This work was carried out under the tenure of a grant from the Dept. of Education, N. Ireland. I would like to thank Dr D.J. Sanderson for his continuing help and motivation.

References Barnes, R.P., Andrews, J.R. and Badham, J.P.N. 1979. Prelimi-

nary investigations of South Cornish melanges. Proc. Ussher Soc., 4, 262-268.

Bott, M.H.P., Day, A.A. and Masson-Smith, D. 1958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Phil. Trans. Roy. Soc., 251A, 161-191.

De La Beche, H.T. 1839. Report on the Geology of Cornwall, Devon, and West Somerset. Mem. Geol. Surv. G.B.

Dearman, W.R. 1969. Tergiversate folds from South-West England. Proc. Ussher Soc., 2, 112-115.

Dearman, W.R., Leveridge, W., Rattey, R.P. and Sanderson, D.J. 1980. Superposed folding at Rosemullion Head, South Cornwall. Proc. Ussher Soc., 5, 33-38.

Dixon, J.M. 1975. Finite strain and progressive deformation in models of diapiric structures. Tectonophysics, 28, 89-124.

Flett, J.S. 1946. Geology of the Lizard and Meneage. 2nd edition, Mem. Geol. Surv. G.B.

Gauss, G.A. 1973. The structure of the Padstow area, North Cornwall. Proc. Geol. Ass. 84, 283-313.

Hendriks, E.M.L. 1937. Rock Succession and Structure in South Cornwall. Q. Jl. geol. Soc. Lond., 93, 322-367.

Hendriks, E.M.L. 1959. A Summary of Present Views on the Structure of Cornwall and Devon. Geol. Mag., 96, 253-257.

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Read at the Annual Conference of the Ussher Society, January 1980

Late magmatic phenomena in the Cornish batholith - useful field guides for tin mineralisation J.P.N. BADHAM

J.P.N. Badham 1980. Late magmatic phenomena in the Cornish batholith - useful field guides for tin mineralisation. Proc. Ussher Soc,, 44-53. The Hercynian Cornish batholith was emplaced at relatively high levels in a zone of little isotatic imbalance. As a consequence it cooled slowly and without interruptions and was not flooded by meteoric waters until relatively late. These relatively uncommon conditions permitted a full range of late- and post-magmatic phenomena to develop, from pervasive metasomatism, through dykes, sheets and breccia pipes, to hydrothermal veins and finally pervasive hydrothermal alteration. The late-magmatic phenomena are characteristic of granites which contain tin - whether the tin be magmatic or in hydrothermal veins, as in Cornwall. They can therefore be used as an important tin prospecting guide in selecting suitable granites within complex magmatic terrains. J.P.N. Badham, Department of Geology, The University, Southampton S09 5NH

Introduction One of the problems facing tin-exploration geologists is the identification of those granites which are potentially fertile in areas otherwise replete with barren granitoids. In particular, while geochemical, petrological and isotopic techniques may serve as useful discriminants, they take time and are not of immediate use in the field. In an attempt to diagnose the field characteristics of stanniferous granites the Cornubian batholith has been studied in some detail and what are considered important diagnostic phenomena are described. Tin mineralisation usually occurs in granites which are aluminous and potassic and commonly contain F, Li and B enriched phases. These granites occur both in the distal parts of continental margin subduction zones and in intraplate settings (Sillitoe 1972). In both environments the granites are characteristically emplaced at high levels (~3kms) and have undergone little or no erosion below their roofs. This implies that the process of generation had little effect on the isostatic balance of the crust (Badham 1976 a,b). Emplacement at high levels requires the rapid ascent of initially relatively dry magma. A lack of water militates against an extensive volcanic carapace-many tin granites were probably roofed by cauldron-collapse structures and relatively thin sheets of felsic pyroclastic rocks.

It is the high-level emplacement, lack of isostatic imbalance and consequent lack of deep erosion that permits relatively slow cooling and relatively uninterrupted continuum of post-emplacement phenomena (Badham and others 1976). These features contrast with those characteristic of porphyry copper environments where relatively wet magmas reaching high levels are associated with thick volcanic piles. There is rapid termination of magmatic processes and a subsequent (after a hiatus) hydrothermal mineralising event. Porphyry copper mineralisation is usually associated with areas of crustal imbalance and consequently of rapid erosion. The analogy between porphyry coppers and some types of tin granites (Sillitoe and others 1975) is not considered particularly apt: the two are considered rather to be end members of a spectrum, possibly linked by the relatively rare porphyry W-Mo-Bi complex bodies of Mount Pleasant type (Dagger 1973; Parrish and Tully 1978). These long-lasting late-magmatic events which may be diagnostic of tin-granites include pervasive metasomatism; dykes, sheets and plugs of various late differentiates; miarolitic cavities and breccia pipes. They are described below from outcrops of the batholith in south Cornwall (Fig. 1) where coastal exposure and extensive mining allow close examination of their natures and inter-relationships.

Figure 1. Location map of south Cornish granites and localities discussed in text.

The St Austell Granite

The various magmatic phases and the important K-H2O, Li and F alterations have been superbly documented by Exley (1958) and little need be added here. However there are numerous smaller scale features of extreme importance. In the Goonbarrow clay pit there is an intriguing pegmatite that has been described at various stages in its exposure during mining operations (Badham and others 1976; Badham and Stanworth 1976). It is highly variable and new exposures show further variations. The pegmatite occupies both vertical and horizontal joints in some areas and elsewhere dips between 40o and 80o. Where vertical it is usually symmetrical and composite; where horizontal it is essentially structureless, and where it dips it is asymmetrical. In many places it separates a megacrystic K-feldspar granite from a medium-grained granite with no megacrysts. However in other places it is either absent and the contact between the two granite types is sharp, or

it is present, but not at the contact. In places the pegmatite has sharp contacts; elsewhere the upper contact is sharp and the lower one diffuse; in other places the contact is marked by "line-rock" (Stone 1975) -although even this is highly variable. In one area the pegmatite has a margin of tourmaline granite between it and the medium-grained granite. In another it consists almost entirely of line-rock with little actual pegmatite. Line-rock also occurs in both the surrounding varieties of granite and is generally parallel to the pegmatite. The pegmatite varies from 0.6m to 5m in thickness but may thin out altogether in places.

Pegmatites of this sort are not uncommon in Hercynian granites and are generally called Stockscheider (stock - a body: scheiden - separating) (Chauris and Lulzac 1973). However the more general names of pegmatite - aplite (Exley and Stone 1964) or of comb-layered - schlieren bodies (Moore and Lockwood 1973) are preferred.

Figure 2. Sketch sections of two stages in the cooling of the St Austell granite cupola. Stage I: growth of the K-feldspar megacrysts in roof zone. Stage II: migration, accumulation and local escape of late magmatic fluids to form a. Comb-layered, schlieren and line-rock structures; b. local pegmatite dykes; c. complex of pegmatite-aplite; d. dyke ofpegmatite and/ or aplite.

The pegmatite liquid was clearly able to behave locally as an independent aqueous-silicate fluid (K-H20-B rich) and elsewhere as a metasomatic fluid. The line-rock and comb-layers are interpreted as areas where late fluids were constrained in their free upward migration and forced along certain pathways (isoviscous lines - or isotherms) (Fig. 2). Dilatancies on these pathways (either cooling joints or perhaps preferentially opened when P fluid > P load) allowed escape to form the symmetrical vertical dyke areas. Miarolitic cavities lined with quartz, tourmaline and K-feldspar, as well as coarse patches of these minerals in the granites represent similar fluids that were unable to escape. In a brief study of fluid inclusions in quartz and apatite (minor) in the pegmatite Barnes (1978) noted two types of primary inclusion. One has commonly a gas bubble and a cubic daughter phase - probably halite - but is essentially liquid. The other, of much higher relief, is homogeneous and appears to be gas-filled. This gas could be the vapour phase of the fluid - i.e. it was boiling, although this is considered unlikely because of the lack of brecciation of the most delicate elongate crystals. The gas could be a separate, immiscible phase of composition as yet unknown. It is concluded then that the pegmatite represents the concentration, migration and partial escape of fluids rich in K20, SiO2, Al203, B and H20. Similar fluids were probably responsible for pervasive K-metasomatism, for topaz-free line-rock, for patch pegmatites and for miarolitic cavities elsewhere in the granite. At Luxulyan, Lister (1978) has now found the famous luxullianite (Bonney 1877) in place and similar rocks

have been seen elsewhere in the St Austell granite and in the Tregonning-Godolphin granite. Luxullianite is the result of replacement of feldspars in granite by quartz and tourmaline. The process is essentially joint-controlled but predates greisenisation. The main chemical changes involve addition of Fe, Al and B and loss of SiO2 (Lister 1978). There is little indication that H20, F or Li were significantly involved. There was therefore a late B-rich metasomatic fluid. At Roche there is a stock of pure quartz-tourmaline rock which, apart from a few miarolitic cavities lined with tourmaline, is homogeneous and structureless. It is not connected with the main granite in outcrop, but must rise above the roof which is only a few tens of metres beneath. Teall (1888) argued that this schorl was a product of complete tourmalinisation of granite. However Hatch and others (1972) saw no palimpsest features and postulated a quartz-tourmaline magma: the writer concurs. The recent discovery of a tourmaline breccia pipe at Wheal Remfry confirms the existence of tourmaline-rich fluids. The elongate pipe cuts megacrystic granite with both tourmaline-rich and ,tourmaline-free line-rock and schlieren. It is cut by quartz-tourmaline vein swarms and greisens. Breccia fragments consist mostly of granite but include some fall-back country rock cemented by tourmaline and less quartz. The breccia is complex and is at present being studied by Alman-Ward at Imperial College. However, the simple mineralogy of tourmaline and quartz and the clear evidence of brecciation show that here the tourmaline-rich fluid escaped the granite explosively in contrast to the apparently passive emplacement of the Roche stock.

Figure 3. Sketch sections of three stages in the emplacement of the Land's End Granite. I. Level one with megacryst growth and local K-H2O segregation in roof. II. Further stoping with tourmaline schlieren and breccia on "faulted" margin: local segregation of B rich phases in roof. III. Further stoping with schlieren on "faults" and development of pegmatite-aplite roof complex.

The Land's End Granite A particularly interesting contact of this granite is exposed in the foreshore from Priest's Cove around Cape Cornwall to Kennidjack. The contact, though essentially steep, is stepped and was clearly generated by stoping. Various late fluids rich in fugitive elements ponded at upper contacts and caused alteration. Some may have partially escaped through the roof. This is well exposed on the foreshore. The granite was first altered by K-metasomatism forming megacrysts and pegmatitic patches; the altered granite was then "intruded" by a small stock of tourmaline granite whose upper parts are feldspar-free. The nature of "intrusion" is not absolutely clear - it would appear, rather that a quartz-tourmaline rich granitic fluid accumulated in and partially domed out of a roof-complex which had just undergone K-H20 metasomatism, but that it failed to separate completely (Fig. 3). At Priest's Cove there is a portion of vertical contact between granite and argillites marked by a tourmaline (quartz) cemented breccia. This has always been described as a fault breccia but is here interpreted as a small breccia pipe formed during stoping (Fig. 3). Tourmaline-lined cavities are not uncommon in granite in these localities, but line-rock is rare. A few aplitic dykes run from granite into the country rocks: they postdate the megacrysts, cavities and tourmalinisation. These exposures demonstrate the existence of K- H2O metasomatism; of partial separation of K - H2O-rich fluid; of tourmaline metasomatism and of partial separation of a tourmaline-quartz fluid. They show that the K - H2O fluids became active prior to those involving tourmaline but that both were effectively co extant: presumably they were essentially immiscible. Hosking (1954) described spatially close but mineralogically varied miarolitic cavities in this area, and concluded that different fluids could segregate in a cooling pluton.

The Tregonning-Godolphin Granite The granite has been thoroughly described by Stone (1975) who perhaps first realised the nature and significance of the superb roof complex. At the western contact at Praa Sands, and particularly beneath a major roof pendant at Rinsey Cove, numerous varieties of 'granite' are juxtaposed with a bewildering complexity. Stone attributes the development of the roof complex to in situ differentiation and volatile streaming within the Tregonning lithionite granite, which itself intruded the slightly older Godolphin granite. The roof complex has been described as a pegmatite-aplite-leucogranite complex. Pegmatites consist predominantly of K-feldspar, quartz, Li-mica with subordinate biotite or tourmaline. Some are homogeneous and structureless: others vary greatly in mineral proportions. Comb-layered feldspars are

abundant and are oriented perpendicular to layering; curvature is generally concave-up the slight dip. Some pegmatites are feldspar-tourmaline-quartz rocks with stellate splays of tourmaline up to 10em long in a structureless matrix. This variety often underlies the comb-layered pegmatite. Leucogranitic areas have a more constant composition of albite, K-feldspar, quartz and lithium mica. They are also homogeneous apart from faint banding and local tourmaline-rich lamellae (schlieren). Aplites also have a relatively homogeneous mineralogy of K-feldspar, albite and quartz. They are frequently banded on both centimetre and millimetre-scales to form all varieties of line-rock. They consistently contain topaz and often apatite as well. All these varieties of granite are interlayered and the layering is parallel in a gross sense to the roof. In general pegmatite occurs at the contact and lining the underside of xenoliths. However in a number of places there are irregular patches of these rock types. The layering is either disrupted by these or gradually loses its definition into them. Different pegmatitic facies, leucogranite and aplite are all superimposed: contacts are indistinct and often gradational over a few centimetres. Time relationships are usually ambiguous, but it would appear in general that pegmatite and aplite slightly postdate leucogranite. These data are interpreted as showing the existence of at least four separate fluids -'normal' pegmatite, tourmaline pegmatite, aplite and leucogranite. The pegmatites developed from K - H20 -B enriched liquids, the leucogranites from Li-rich granite and the aplites from a F-rich (gaseous?) medium. The banding in the line-rock is essentially the product of diffusion reactions in the "solid" state (Stone 1975), although disruptions of the banding show that the "solid" could still move. The segregations of the two different pegmatite varieties from each other and from the aplite imply some sort of immiscibility. High viscosities coupled with strongly contrasting chemistries inhibited complete separation and resulted in diffuse boundaries. Volatile contents were at no time high enough to drive these fluids out of the granite, upwards either as intrusions or as breccias. However in some areas vertical steps on the contact have tourmaline-schlieren parallel to them and tourmaline-filled cracks close by implying limited upward mobility of B-rich material through partially solidified granite. At Megilligar rocks and along Tremearne cliff a-series of granitic sheets wedges outward from the roof zone of the granite, dipping away from the contact and thinning out in about 1 km (Fig. 4) (Stone 1969). The sheets ramify in a complex manner forming a network that cross-cuts the cleavage in the country rocks by a few degrees. Near the contact the sheets are homogeneous and composed essentially of leucogranite. Some 400m out they start to develop line-rock - with thin bands rich in topaz and some in tourmaline - and pegmatitic patches. Gradually

Figure 4. Sketch view from the sea of the Tremearne cliff and Megilligar rocks pegmatite-ap(ite sheets, showing change in nature away from granite and the approximate location of sill F.

the differentiation increases until complex banded sheets of lined aplite, leucogranite and pegmatite make up the whole of each sheet. In the last 100m of exposure the sheets thin rapidly and consist predominantly of quartz (Hosking 1954). The sheets are clearly intrusive and Stone (1969) argued that the complex banding developed essentially in place by solid state diffusion within a magma that had undergone lateral differentiation, if only in terms of volatile content. That diffusion had indeed occurred is undoubted and this has allowed segregation of essentially an aqueous-potassic phase (pegmatite) (with only minor tourmaline) from a F-Li-P-rich material (aplite). However it is not clear that the banding and segregation are solely the result of in-place diffusion. Amongst the main sheets there is one only 50cm thick (Fig. 5). This generally dips gently but in places steps up a few tens of cms and then to the east, turns up the cliff with a dip of 70°. In its flat-lying portions it has a pegmatitic top with pendant feldspars, line-rock in the centre and fine structureless aplite at the base. This fine structure becomes confused at the small steps, but the intrusion becomes symmetrical where it turns upwards. The margins here consist of horizontal and locally upward curved feldspars and the centre of line-rock and aplite. These data imply the existence of two fluids in the intrusion whose differing properties kept them essentially separate and refute the hypothesis of purely in situ diffusion. The degree of post-emplacement diffusion is shown by the line-rock and the diffuse boundaries between pegmatite and aplite. Using this sheet as the type

'simplest' example, the complex sheets are interpreted as the product of numerous separate intrusions of this two-liquid material, as was partially suggested by Hosking (1954). The presence of xenoliths of line-rock and of pegmatite included in later bands and of cross-cutting relationships support this. Tingey (1977) collected samples across the same 2m-thick sheet where it was leucogranite and where it was segregated into pegmatite-aplite (Fig. 4). The samples were analysed separately: however the averages of the analyses at each point should give a fair estimation of the

Figure 5. Detailed sketch of the principal features of sill F.

bulk composition at each point (Table 1). For major elements there is no significant difference between the two average analyses - although the scatter, particularly for alkalis, is obviously greater in the pegmatite-aplite. The results also compare closely with Stone's (1975) analysis for Tregonning leucogranite (Table 1), justifying his proposition that the sheets were formed in the same way as the roof complex of the granite. For the few trace elements analysed the two sets of data show again little difference except for arsenic. Arsenopyrite and loellingite (Stone, personal communication 1979) are common accessories in the pegmatites. Arsenic would appear to have been concentrated by lateral differentiation. However the other data imply that the leucogranite and the pegmatite-aplite are chemically identical and are not the product of progressive differentiation. Differences must then be due to some physical control. It is proposed here that a fluid escaped from the roof of the Tregonning granite cupola and migrated rapidly, and in a series of pulses, outwards into the hot country rocks along propagating fractures-(Hosking 1954; Moore 1975). At higher temperatures the fluid crystallised as leucogranite. Further out at lower temperatures two fluids (one essentially aqueous and the other volatile and magmatic) were able to separate and crystallise separately. The chemical incompatibilities and high volatile contents allowed post-emplacement diffusion

reactions. The spent fluid containing only silica precipitated quartz distally. This spent fluid may well have been essentially hydrothermal rather than magmatic and We may be seeing the continuum from magmatic to hydrothermal in this section. A study of fluid inclusions is planned. Cligga Head The nature and structure of the small stock of altered megacrystic granite has been thoroughly described by Moore and Jackson (1977). However they and previous authors fail to mention some of the variability in the granite particularly at the roof and near the southern contact. In these areas line-rock and lensoid pegmatites with pendant feldspars are locally developed. They parallel the contact and also the alignment of K-feldspar megacrysts (often incorrectly called fluxion-banding). In places the line-rock and pegmatite break down to rather diffuse chaotic areas of patches of both in a leucogranitic matrix. These areas dome upwards out of the line rock areas into more homogeneous granite. They are interpreted as igneous breccias, but which were formed of a material that was so caustic and at a time when the host

Table 1. Partial chemical analyses for rocks of the Tregonning Granite

% Tregonning

Granite Leuco-granite

Leucogranite West Megilligar

Pegmatite/ Aplite East Megillar

% % % σ % σ

SiO2 71.10 72.00 71.40 ± 1.62 71.30 ± 1.23 TiO2 0.30 0.03 0.03 ± 0.004 0.04 ± 0.001 Al2O3 15.20 15.97 17.10 ± 0.23 16.70 ± 0.27 Fe2O3 2.27 0.64 0.66 ± 0.05 1.04 ± 0.25 MgO 0.40 0.04 0.19. ± 0.06 0.21 ± 0.13 CaO 0.57 0.32 0.23 ± 0.08 0.31 ± 0.18 Na2O 2.18 4.59 4.30 ± 0.60 3.90 ± 0.62 K2O 6.25 3.91 3.70 ± 0.25 3.85 ± 0.55 Total 98.27 97.50 97.61 97.35 ppm Ni 29 ± 1.7 22 ± 3.9 Cu 13 ± 1.9 10 ± 2.3 Zn 20 ± 5.1 21 ± 6.8 Ga 49 ± 4.6 47 ± 5.6 As 3 ± 1.9 198 ± 278 Pb 8 ± 7.0 4 ± 1.7 Granite and Leucogranite from Stone (1975). Megilligar analyses - averages and standard deviations for 5 samples at two separate locations 100m apart - from Tingey (1977) - see text.

was so plastic that they have "eaten" rather than "blown" their way upwards. They gradually fade out within a few metres. They appear to have involved F-Li-rich granitic fluids and post-date the K-H20 rich pegmatites and F-Li rich line-rock.

The southern contact of the stock has been described as a fault (Moore and Jackson 1977). The dip of aligned megacrysts at about 70o is parallel to the contact over a zone about 80m wide. North of this it changes to near horizontal over an internal discontinuity, also described as a fault. This southern boundary zone contains a far higher proportion of late magmatic variants than does the main stock and all are aligned or elongated Parallel to the contact. Dykes of aplite and leucogranite cross the contact into hornfelsed slates but appear to have arisen from the contact zone areas - they do not definitely crosscut the magmatic rocks. This alone casts doubt on the significance of faulting sensu stricto (i.e., brittle failure after crystallization). This contact zone is seen rather as a stoped contact up which numerous late magmatic fluids escaped or partially escaped. The faults were originally schlieren produced by upward movement of late magmatic material between more viscous granite and the wall rocks. There were clearly later failures on these zones after crystallisation but these are not thought to have any great spatial significance.

The Cligga stock shows the effects of K- H20- Li-F fluids, with only the beginnings of segregation into pegmatitic and aplitic fractions. Boron was of minor significance in the magmatic stages but is abundant in the hydrothermal veins. Conclusions The Cornish batholith was emplaced by stoping and the individual cupolas rose out of the roof of the main body by further stoping of more differentiated material (Exley and Stone 1964). Final emplacement almost certainly took place in a near-solid state. All the granites presently exposed show evidence of more than one phase of intrusion, but in many the later phases appear to have close genetic links with the alteration of their precursors. Exley and Stone (1964) show how aqueous and K-rich fluids rose up through the earlier granites causing prevalent K-metasomatism and growth of K-feldspar megacrysts. Stone (1975) argues that at the same time late magmatic differentiates rich in Li, B and F caused both pervasive alteration of earlier crystallised material and generation of new melts (metasomatic anatexis). This phenomenon is caused by the change in bulk composition of hot granite by metasomatism such that the new composition has a melting point below the temperature of the 'solid' parent. It requires, obviously, very slow cooling. Slow cooling is not, however, synonymous with slow crystallisation. On the contrary, since compositions were always close to eutectic, complete crystallisation would have been rapid.

It has been shown that a number of different fluids may have co-existed in the cooling tops of tin-bearing granites. These liquids may have been the remnants of primary crystallisation 'and strongly enriched in fugitive elements. They may also have been newly generated metasomatically by escaping volatiles. These liquids were enriched particularly in H20, K, F, B and Li. The most frequent associations among these elements appear to be: H2O- K; H20-K-B; B; Li-F and F. The H20-K association is always distinct from the F association, but the others may be mixed.

These various fluids appear to have had properties different from each other, varying properties with changing temperature and different potential effects on their host rocks. Some of the fluids were essentially aqueous (K - H20 types--pegmatites, etc); some were gaseous (F - rich types -- aplites); and some were magmatic. For example the leucogranite was clearly formed from a magma which, in the roof complex, partially segregated from a B- rich phase and, in the Megilligar sheets, clearly unmixed into immiscible pegmatite and aplite phases. The segregation of B-rich magma from granitoid magma is clear also at Cape Cornwall and in the St Austell area. Perhaps the only magmatic type not described from Cornwall but noted as a late immiscible phase in stanniferous granites elsewhere is topazite (e.g. Eadington and Nashar 1978): it certainly should be present and one might predict its discovery.

There is some debate as to whether these late magmatic, aqueous and gaseous fluids are purely the result of extreme differentiation and unmixing or whether some at least are the products of metasomatic anatexis. The addition of a few per cent of H20, Li, F or B lower the melting temperature of granite significantly (e.g. Tuttle and Bowen 1958; Wyllie and Tuttle 1964; Manning 1979; Pichavant 1979; Charlton and Martin 1978). Manning argues that his experimental data indicate an origin by differentiation and that there is no need for generation of these late fluids by anatexis as argued by Stone (1975). The field data generally support Manning's arguments in that they show migration and accumulation of these fluids upward into the granite roofs and there is little field evidence for anatexis. However the process Stone proposes would have taken place on a rather larger scale and the vertical exposure necessary to prove his case is not available.

It is implicit from the descriptions above that while much of these late magmatic fluids accumulated in place some at least migrated upward through cooling granite to concentrate in and above the roof. The manifestations of passage of such material could be many and various depending on total pressure, temperature and chemical and physical contrasts between it and its host. For any one migrating fluid the effect of its passage may be different at different palaeodepths. The fluids may have remained trapped in the granite (miarolitic cavities); may have diffused pervasively through it (metasomatism);

may have migrated and accumulated within or beyond it (aplite-pegmatite complexes, line-rock and roof complexes); may have escaped up joints (dykes) or may have blown their way out (breccia pipes). The upper contact of the granite (roof) acted as a primary barrier to upward migrating fluids, presumably because of early chilling and loss of permeability. This trap was locally breached both chemically and physically and limits to ascent may have been in the overlying metasediments or even the surface. However as the intrusion cooled so the properties that allowed the original contact to act as a barrier migrate inwards and downwards. These changes in properties in the crystallising pluton will be parallel not to the intrusive contacts but to the isothermal surfaces. Late fluids may find internal 'roofs' which impede their process. If these are horizontal then the fluids will pond and crystallise. If they slope then the fluids may migrate up-dip forming comb-layered sheets and either pond higher up or escape into early cooling joints. The evolution of crystallisation of these fluids overlapped initially with the crystallisation of the roofs of the host intrusions and finally with the pervasive hydrothermal process. In general K-metasomatism predated Li, B and K - H20 fluids which in turn predated F-rich fluids. Potassium ceased to be an active ingredient before the hydrothermal systems were fully operative, but F, Li and B were intimately involved with these too. These then are the late magmatic features closely associated with tin mineralisation. Tin may be taken up by biotite preferentially during crystallisation of granite or may be concentrated as a fugitive element in the last liquids. In the latter case primary concentrations of tin will be magmatic and cassiterite will be found in the various late magmatic rocks. Such is indeed the case in the Bushveldt complex (Groves and McCarthy 1978) and in many of the European Hercynian granites (e.g. Chauris and Lulzac 1973). However such first-stage concentrations are not usually of great economic significance. Cassiterite is notably absent from late magmatic rocks in Cornwall - with the exception of the Halvasso pegmatites (Jackson 1978). Presumably it had been earlier locked up in biotite and was released only during later pervasive hydrothermal alteration. This late release' resulted in far more concentrated, and therefore more valuable, vein-type mineralisation. Thus not only are the late magmatic rocks described here useful field guides for stanniferous granites: the presence or absence of cassiterite in them may determine prospecting philosophy and ultimate economic potential.

Acknowledgements. t am most grateful to Dr M. Stone for discussions and critical comments on an earlier draft. Dr D Manning kindly made preprints of his work available as did Dr B. Charoy, Since this paper was prepared Dr Charoy's thesis, in which many of the same phenomena are described and similar conclusions reached, was published: the reference is included below. I am most grateful also to the management of E.C.L.P. for continued access to their workings. References Badham, J.P.N. 1976a. Orogenesis and metallogenesis with

reference to the silver-nickel-cobalt arsenide ore association, Geol. Assoc. Can. Sp. Paper, 14, p.559;

Badham, J.P.N. 1976b. Cornubian tectonics - lateral thinking. Proc. Ussher Soc., 3, 558-62.

Badham, J.P.N. and Stanworth, C.W. 1976. The curved-crystal pegmatite, Goonbarrow. Proc. Ussher Soc., 3, 44148.

Badham, J.P.N., Stanworth, C.W. and Lindsay, R.P. 1976. Post-emplacement events in the Cornubian Batholith. Economic Geology, 71,534-39.

Barnes, R.P. 1978. The Goonbarrow pegmatite: a petrographic and geochemical investigation. Unpubl. student thesis. Univ. Southampton, 49pp.

Bonney, T.G. 1877. On the microscopic structure of Luxullianite. Min. Mag., 1, 215-221.

Charoy, B. 1979. Deflation et importance des phenomenes deuteriques et des fluides associes dans les granites. Conse-quences metaltogeniques. Mem. 37. Ann. lab. Sci Terre. Universite de Nancy, 364pp.

Chauris, L. and Lulzac Y. 1973. Les aplites a topaze et les Stockscheider du leucogranite de Scaer (Finistere). Bull. Soc. geol. mineral. Bretagne, 1, 21-30.

Chorlton, L.B. and Martin, R.F. 1978. The effect of Boron on the granite solidus. Can. Mineral., 16, 239-244.

Dagger, G.W. 1973. Genesis of the Mount Pleasant W-Mo-Bi deposit, New Brunswick, Canada. Trans. I.M.M., B73.

Eadington, P.J. and Nashar, B. 1978. Evidence for the magmatic origin of Quartz-Topaz rocks from the New England Batholith. Australia. Contrib. Min. Petrol., 67, 433-438.

Exley, C.S. 1958. Magmatic differentiation and alteration in the St Austell Granite. Quart. Journ. Geol. Soc., 114, 196.

Exley, C.S. and Stone M. 1964. The granitic rocks of south-west England In Hosking, K.N.G. and Shrimpton, G.J. (eds.) Present views of some aspects of the geology of Cornwall and Devon. Trans. R. Geol. Soc. Cornwall, 150th Anniv. Vol., 131-184.

Groves, D.I. and McCarthy T.S. 1978. Fractional crystallisation and the origin of tin deposits in granitoids. Min. Dep., 13, 11-26.

Hatch, F.H., Wells, .A.K. and Wells, M.K. 1972. Petrology of the igneous rocks, 13 Edn. 551pp Thomas Murby & Co. London.

Hosking, K.F.G. 1954. Cornish pegmatites and bodies with pegmatitic affinities. Trans. Roy. Geol. Soc. Conrwall, 18, 411.

Jackson, N.J., 1978. The Halvosso Pegmatites. Proc. Ussher Soc., 4, 190-192.

Lister, C.J. 1978. Luxullianite in situ within the St Austell granite, Cornwall. Min. Mag., 42, 295-7.

Manning, D.A.C. 1979. An experimental study of the effect of fluorine in addition to water, on crystallisation in the system Qz-Ab-Or, and its application to Cornish granitic rocks rich in fluorine. Proc. Ussher Soc:, 4, 380-389.

Moore, J.G. and Lockwood, J.P. 1973. Origin of Comb layering and Orbicular structure, Sierra Nevada Bathotith, California, Bull. Geol. Soc. Amer., 84, 1-20.

Moore, J. McM. 1975. A mechanical interpretation of the vein and dyke swarms of the S.W. England Orefield. Min. Dep., 10, 374-88.

Moore, J.McM. and Jackson, N. 1977. Structure and mineralisation in the Cligga Granite Stock, Cornwall. Journ. Geol. Soc. Lond., 133, 467-480.

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Sillitoe, R.H., Halls, C. and Grant, J.N. 1975. Porphyry tin deposits in Bolivia. Econ. Geol., 70, 913-927.

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Read at the Annual Conference of the Ussher Society, January 1980

Ore genesis and controls of mineralisation in the Upper Palaeozoic rocks of north Devon R.C. SCRIVENER M.J. BENNETT

Introduction In the course of the recent geological survey of the Ilfracombe (277), Bideford (292) and Barnstaple (293) sheets, the opportunity was taken to carry out a field and literature study of the ore deposits in those districts and, in particular, to seek evidence of ore genesis and of stratigraphical and structural controls. Previous work in the district has not been extensive in comparison with the voluminous literature on the ores of Cornwall and south Devon;-this reflects the generally small size of the deposits and their limited output. Dines (1956) gave a detailed account of the mines based on 19th century mining records; Rottenbury (1974) has made an exhaustive study, particularly of the mining history and mineralogy of the deposits, and Cookes (1947) recorded available historical and other information about the Combe Martin mines. Other papers have included an account of South Molton Consols (Badham and others 1979) and the results of isotopic age determination on clays associated with the mineral deposits (Ineson and others 1977). In the following account, notes on the mineralisation in the Devonian and Lower Carboniferous formations of north Devon are followed by a general discussion. Lynton Slates and Hangman Grits The oldest rocks of the district are comparatively lacking in ore deposits. Sites of a few scattered unsuccessful mining trials in the marine Lynton Slates and continental

lower Hangman Grits are known. Ore bodies in the paralic upper beds of the Hangman Grits were formerly worked for iron, but little is known of the nature of these deposits, which lie to the north-east of Combe Martin and extend discontinuously inland from the coast for a distance of about 3km. Specimens from the old open-workings are of veins of goethite in ferruginous sandstone. A statement by Smyth (1859) that these deposits were bedded nodular ironstones has not been substantiated. Ilfracombe Slates and Morte Slates The Ilfracombe Slates have been divided into four formations in upward order: the Wild Pear Slates, Lester Slates and Sandstones, Combe Martin Slates and Kentisbury Slates (Edmonds and others, in preparation). Apart from impregnations of hematite in the sandstones of the paralic Wild Pear Slates, significant metal deposits are restricted to the marine Lester Slates and Sandstones. Argentiferous lead ores were formerly worked from this formation in the vicinity of Combe Martin, and the history of these operations, which commenced in the Middle Ages, has been documented by Cookes (1947) and Rottenbury (1974). The orebodies are not. cross-cutting hydrothermal veins of the type familiar in Cornwall and south Devon, but irregular reef-like masses of quartz and siderite with base-metal sulphides developed in the cores of near-isoclinal overturned folds in the slates. The principal sulphides are galena and blende, with minor chalcopyrite and pyrite. Tetrahedrite, millerite and stibnite have also been recorded, but were not found in the course of this survey. The silver content

R.C. Scrivener and M.J. Bennett 1980. Ore genesis and controls of mineralisation in the Upper Palaeozoic rocks of north Devon. Proc, Ussher Soc., 5, 54-58. The mineralisation in the Devonian and Lower Carboniferous rocks of north Devon is described. Significant mineralisation is restricted to argentiferous galena in the Lester Slates and Sandstones, siderite, goethite and limonite in the Pickwell Down Sandstones and galena, blende and chalcopyrite in the Pilton Shales. Ore textures, the distance from the Cornubian batholith and the degree of stratigraphical control, suggest that the ores in these rocks have been formed by re-mobilisation of syngenetic concentrations of base metals. In addition exhalative volcanic fluids may have contributed to the deposits, particularly in the Pilton Shales and Lester Slates and Sandstones. R.C. Scrivener and M.J. Bennett, Institute of Geological Sciences, St Just, 30 Pennsylvania Road, Exeter EX4 6BX

of the lead ores is known to have been high and Dines (1956) recorded 0.28kg per tonne, but no silver minerals have been encountered apart from a specimen of a filament of native silver in galena, presumably from the zone of enrichment. The sulphides are concentrated in a unit of mudstone which is visible in the coastal exposures of Lester Slates and Sandstones at Lester Rock, and to the west of Combe Martin. Aggregates of galena and sphalerite, with traces of chalcopyrite and pyrite, are streaked out along the cleavage of this bed. Similar, though less well developed stratabound sulphide bodies occur at other horizons within this formation and traces of galena and sphalerite are present in the limestones of the overlying Combe Martin slates. The Kentisbury Slates are sparsely mineralised but a trial for copper known as Wheal Eliza lies close to the boundary with the Morte Slates. Ore from this mine consists of siderite with disseminated chalcopyrite and pyrite. Goethite and hematite are also present. Extensive veins of iron ore are present in the eastern part of the outcrop of the Morte Slates and include the Deerpark Mine and Exmoor Mine complexes. They are considered to be hydrothermal in origin. The principal ore species are siderite, goethite and hematite, in a quartz gangue. These veins are normally conformable with the bedding of the enclosing sediments.

Pickwell Down Sandstones Siderite veins are common in the epicontinental Pickwell Down Sandstones. Secondary processes have altered much of the ore to goethite and limonite and also, in places, have led to concentrations of manganese ores. In the eastern part of the outcrop, along the southern limb of the Bray Valley Anticline, veins in the vicinity of Heasley Mill carry significant quantities of Copper. They are further distinguished by the presence of the specular variety of hematite. Stratigraphical control within the formation is not marked, though the copper-iron deposits lie close to the boundary with the overlying Upcott Slates. Unlike the orebodies in the Morte Slates, the veins in the Pickwell Down Sandstones are not generally conformable with the bedding but have cross-cutting relationships with the enclosing strata. Upcott Slates and Baggy Sandstones A considerable range of ores have been recorded from the Upcott Slates, which are lagoonal and coastal swamp sediments, but the mineralisation is not well developed apart from the vein of baryte at Bentwichen, which was

discovered and described by Rottenbury (1974). This is a cross-cutting vein with a strike length of almost l km. Barytes and quartz are the principal components, with minor iron oxides and some copper sulphides. The lode is nearly vertical and ranges up to 2m in width. Rottenbury also noted specimens of baryte, chalcopyrite and tetrahedrite on the dumps of the nearby Brittania Mine, a 19th century trial for copper and iron which was found to carry traces of gold. Other trials for copper exist in the Upcott Slates and at Upcott Mine, in the eastern part of the outcrop, copper was found in association with lead and zinc. The Baggy Sandstones are for the most part barren, apart from traces of copper and lead proved at isolated trials. Pilton Shales The Pilton Shales span the Devonian-Carboniferous boundary and form a broad outcrop. They are marine shales, commonly though not invariably calcareous, with locally well-developed sandstone horizons and some thin limestones. Mineralisation is widespread in these rocks and has been exploited at a number of localities. The deposits are veins which carry galena and blende with siderite, chalcopyrite and other sulphides. Minor quantities of minerals bearing arsenic and antimony are present at a number of localities and seem to be characteristic of the formation. Badham and others (1979) have described an assemblage, including arsenopyrite, tetrahedrite, pyrite, marcasite, galena, sphalerite and chalcopyrite, at South Molton Consols, one of the larger workings of this group. In every case these veins are fracture-fillings, with much included brecciated wallrock, extensive argillic alteration of the enclosing sediments and textures attesting their hydrothermal origin. In this they differ from the lead-zinc deposits at Combe Martin. Examination of borehole material has shown local impregnation of sulphides, particularly pyrite, and these are not restricted to any particular horizon. It is likely that the mineralisation is derived from mobilisation of these syngenetic concentrations. The Carboniferous strata younger than the Pilton Shales are practically barren and no significant deposits have been exploited from them. Discussion and conclusions Lead and zinc ores are found in the Lester Slates and Sandstones and also in the Pilton Shales. Both formations are similar in their depositional setting, being sequences of shallow-water marine mudstones with sandstone and minor limestone intercalations which are underlain by considerable thicknesses of paralic and

epicontinental sediments. This type of depositional environment is known to be a host to stratabound ore deposits and there is a similarity with the occurrences of the base metal ores in the Rheinisches Schiefergebirge. Thus there is reason from both formations to believe that the orebodies were formed as a result of the mobilisation of syngenetic concentrations of base metals. In the case of the Lester Slates and Sandstones the mineralised horizon is a single unit of mudstone, while several horizons contribute to the mineralisation in the Pilton Shales.

Exhalative fluids associated with contemporaneous volcanism may have been partly responsible for the syngenetic metal concentrations. Goldring (1957) identified tuff bands in the Lower Pilton Shales and, while there is no record of volcanic rocks in the Lester Slates and Sandstones, the stratigraphically equivalent Avill Group of the Quantock Hills (Webby 1965) contains a substantial pyroclastic member, the Cockercombe Tuff. An igneous influence of this kind might explain the unusual but characteristic occurrences of arsenic and antimony in the veins of the Pilton Shales, and the high levels of silver in the Combe Martin ores.

The sideritic and locally copper-rich ores of the Pickwell Down Sandstones are also considered to have been derived from syngenetic concentrations which accumulated in the predominantly continental 'red-beds'. The base of the formation is marked by a discontinuous, but locally thick tuff band, the so-called Bittadon Felsite, and exhalative fluids associated with this contemporaneous volcanism may have contributed to the metal content of the sediments.

The tendency of the competent Pickwell Down Sandstones to fracture rather than deform under tectonic stress, and to fracture the adjacent sediments, produced channels for mineralisation that extend into the Morte Slates and Upcott Slates. This may account for the presence of iron and copper veins in those rocks.

The source of ore-forming fluids responsible for the north Devon mineral deposits has been the cause of some discussion. Dines (1956) considered that the ores represented the outermost zone of the low-temperature hydrothermal activity associated with and derived from the Cornubian granite batholith. More recently, Ineson and others (1977) have speculated that fluids from this batholith migrated along hypothetical low-angle fractures to feed the north Devon mineralised structures during the Hercynian orogeny. The distances involved and the absence of mineralisation in the extensive intervening tracts of Upper Carboniferous rocks do not support these views. It is considered that these mineral deposits were emplaced as a result of the circulation of fluids generated by low-grade regional metamorphism during the Hercynian earth movements.

Syngenetic accumulations of metals in the marine and epicontinental shale formations, and the continental and

paralic sandstone formations are considered to be the principal source of the metal ores. Stratigraphical control is marked in the case of lead-zinc deposits, but less so in the case of iron ores. Fracturing associated with large-scale folding in the competent Pickwell Down Sandstones is considered to have controlled the deposition of the copper-iron deposits of the Heasley Mill area and to account for the distribution of exotic assemblages in the Upcott Slates. Acknowledgements. The authors wish to thank Messrs G. Bisson, E.A. Edmonds and K.E. Beer for their constructive criticism of the text. This contribution is published by permission of the Director, Institute of Geological Sciences. References Badham, J.P.N., Cosgrove, M.E., Crump, D.R., Edwards, P.J., Leach, A., Leclere, M., Saunders, R.A. and Winter, A. 1979. Geochemical and ecological investigations at the South Molton Consols mine site. Proc. Ussher Soc., 4, 449-466. Cookes, H. St. L. 1947. The Silver-Lead Mines of Combe

Martin, Devon. Unpublished Report. Dines, H.G. 1956. The Metalliferous Mining Region of South-

West England. Mem. Geol. Surv. Gr. Br. Edmonds, E.A., Williams, B.J. and Whittaker, A. Geology of

the country around llfracombe and Barnstaple. Mem. Geol. Surv. Gt. Br. (in preparation).

Goldring, R. 1957. The last toothed Producteilinae in Europe (Brachiopoda, Upper Dev). Pa#iont. Z., 31, pp.207-228.

Ineson, P.R., Mitchell, J.G. and Rottenbury, F.J. 1977. Potassium-argon isotopic age determinations from some North Devon mineral deposits. Proc. Ussher Soc., 4, 12-23.

Rottenbury, F.J. 1974. Geology and mining history of the metalliferous mining areas of Exmoor. Unpubl. Ph.D. thesis Univ. Leeds.

Webby, B.D. 1965. The stratigraphy and structure of the Devonian rocks in the Quantock Hills, West Somerset. Proc. Geol. Assoc., 76, pp.321-343.

Written discussion Dr C.M.L. Bowler: Have the authors had the opportunity to extend their interesting work westwards into the area of predominantly Devonian clastics of SW Cork, Republic of Ireland, where early workers of the Irish Geological Survey recognised a thin but very extensive bed of copper-bearing sandstones? Most of the small 19th century copper mines of this part of Ireland appear to have been based on coarser minerali-sation located in quartz veins which generally strike approximately north-south cross-cutting the bedding. These veins are of limited extent and probably arose by lateral secretion into low pressure zones. Authors' reply: Regrettably, the authors have not been able to extend this work beyond SW England. A comparison of these areas might prove to be interesting and would provide a useful field for further research.

Dr J.P.N. Badham: The authors' contention that the mineralisa-tions in N, Devon are syngenetic is strongly supported by Rottenbury's work and by that of students and myself at Southampton. We also agree that various - principally metamorphic/tectonic - concentrator events have operated on these mineralisations to form the bodies that were actually mined. However, I find it hard to be sympathetic with the authors' contention that the mineralisations are of volcanic-exhalative origin, for the following reasons:

1. The connection with volcanism is tenuous in the extreme. 2. Precipitation and preservation of exhalative sulphides requires low Eh and still submarine conditions. The miner-alised beds were formed in shallow, commonly oxidised waters, show abundant evidence of current activity, and contain an abundant infauna. 3. The assemblage of elements in the deposits (Fe-Pb-Zn-Cu-Sb-Cd-As-Se-Ba, with low Ni and Co) is dissimilar to that in volcanic exhalative ores and more similar to that in so-called "red-bed" syn/diagenetic deposits.

Authors' reply: The authors do not consider the connection with volcanism to be tenuous in the extreme. It is felt to be significant that the three most extensively mineralised stratigraphical units, the Lester Slates and Sandstones, Pilton Shales and Pickwell Down Sandstones are also associated with more or less distal, though well marked contemporaneous extrusive igneous activity. To what extent volcanism has contributed is, of course, open to question. It is accepted that much of the mineralisation, in particular the iron-copper deposits of the arenaceous rocks, requires no exhalative association.

Read at the Annual Conference of the Ussher Society, January 1980

Field relations and geochemistry of the foliated granitic sheets of Sark, Channel Islands G.M. POWER W. GIBBONS

Introduction This paper examines the field relationships and geochemistry of a suite of Precambrian foliated quartz dioritic to granitic sheets which intrude ancient semi-pelitic, amphibolitic and augen gneisses on Sark. Little detailed petrography and no systematic geochemistry has been published on these intrusions which have been attributed to early Cadomian igneous activity on the basis of an imprecise Rb-Sr whole rock isochron age of 650 + 90 m.y. (Adams 1976). Gibbons and Power (1975) observed that the gneissic country rock had undergone at least two phases of intense deformation before the emplacement of the granitic sheets and concluded on all the evidence available that the gneisses were most probably Pentevrian in age. Field relations The granites occur as a series of sheets at different structural levels which broadly follow the dominant foliation in the gneisses but locally cross-cut it. For the purpose of this paper they have been divided into three groups on the basis of their geographical location (Figure 1): the Little Sark sheet, the Central Sark sheets and the Northern Sark sheet. Two other sheets occur on the nearby island of Brecqhou just west of Sark and are probably an extension of the Little Sark intrusion.

A walk from Creux Harbour (478757) southwards along the coastal path to Hogsback (469749) is instructive in demonstrating some of the features common to the granites and in particular to the Central Sark Sheets. At Creux Harbour the granite is granodioritic in composition. It has a foliation striking N-S and dipping west at about 250. Hornblende prisms define a shallow northward-plunging lineation. The base of the sheet is exposed on Les Burons, the rocky islet opposite the harbour whilst the top of the sheet may be seen in the cliff at the back of Creux Harbour indicating a vertical thickness of about seventy metres. It is overlain by amphibolitic gneiss, near the base of which lies a series of ultrabasic lenses (Sutton and Watson 1957). The path south along the cliff-top above the harbour follows the contact between foliated granodiorite and amphibolitic gneiss (Gibbons 1975). The contact is always sharp and, although broadly following the structure of the gneissic envelope, the granodiorite does locally vein into and cross-cut the amphibolitic foliation. A detour to the foot of the cliff at Les Laches (477755) demonstrates the foliated intrusion to be slightly move leucocratic away from the amphibolite contact. Unlike the Little Sark and Northern sheets, enclaves are rare and where they do occur are recognisable as partly assimilated rafts from the overlying amphibolite. A fine example of one such xenolith lies slightly above HWM west of Les Laches (4765 7544 - low tidy only). The strong foliation in the surrounding granodiorite passes completely through the amphibolitic xenolith.

G.M. Power and W. Gibbons 1980. Field relations and geochemistry of the foliated granitic sheets of Sark, Channel Islands. Proc. Ussher Soc., 5, 59-67. Foliated quartz dioritic and grandioritic sheets, dated as early Cadomian (late Precambrian) intrude Pentevrian gneisses on Sark. They contain a variety of enclaves, some recognisable as partly assimilated country rocks, some probably brought up from lower crustal levels. The strong granitic foliation, cut by pegmatitic veins, and its orientation governed by the shape of the envelope, appears to have been developed about the time of magma consolidation. A range of late alteration reactions have modified the original mineralogy of the sheets and a complex thermal history is indicated. Chemical data show a calc-alkaline series with each of the main sheets forming only part of the full trend. Contamination by reaction with amphibolitic country rock is demonstrated by anomalously high MgO, Cr and Ni values in some sheets. The scatter on SiO2 variation diagrams for the alkali and rare earth elements may reflect the late alteration. G.M. Power and W. Gibbons, Department of Geology, Porstmouth Polytechnic, Burnaby Road, Portsmouth POI 3QL

Figure I. Outline geological map of Sark. 1. Semi-pelitic gneiss; 2. Amphibolitic gneiss; 3. Augen gneiss; 4. Foliated quartz diorite and granodiorite. CH: Creux Harbour; DP: Derrible Point: HB: Hogsback; LS: Little Sark; GG: La Grande Greve; PS: Port es Saics: TB: Telegraph Bay: PM: Port du Moulin: LaG: La Grune.

A particularly clear intrusive contact is exposed on the east side of Derrible Point (4745 7500). Although the contact is sharp, slightly cross-cutting granitic patches may be seen growing in the overlying amphibolite and signs of partly assimilated gneiss occur as ghost structures in the foliated granodiorite. The granitic sheet at Port du Moulin (459773) on the west coast of Sark shows many similarities to the Creux Harbour sheet, again containing rafted xenoliths of the overlying amphibolite. It has been suggested on structural grounds that both may be parts of the same sheet (Sutton and Watson 1957), At the tip of Hogsback a granitic sheet at a higher structural level than the Creux Harbour sheet has intruded and completely disrupted the thick amphibolite horizon. It is clearly younger than the amphibolite. Enclaves of amphibolite of all sizes ranging from millimetres to metres are enclosed in the granite and may be seen in various stages of assimilation. Reaction between granite and amphibolite is shown by the development of hornblende at the margins of the enclaves. This Hogsback type of relationship may also be seen in other sheets, particularly the fault-bounded exposures on the west coast at Telegraph Bay (456766) and Port es Saies (458751). Virtually the whole of Little Sark is composed of one large granitic sheet with a N-S striking westerly-dipping foliation. Away from the gneissic contact, the foliation tends to be less strongly developed, grading into a strong, northerly-plunging mineral lineation. The sheet is quartz dioritic in composition and taken as a whole is the most basic on Sark. However, there is a notable exception to this generalisation at La Grande Gréve (457744) where close to the contact with the semi-pelitic gneisses the rock is particularly acid in composition. There is a recognisable contact between quartz diorite and a seven metre wide band of leucocratic rock and while the quartz diorite is full of enclaves this band contains very few. It may be a thick granitic vein emplaced at a late stage along the country rock contact of the quartz diorite. The Little Sark quartz diorite is characterised by abundant enclaves of different types in various stages of assimilation. There are those which are obviously country rock gneiss both of semi-pelitic and amphibolitic composition. In addition there are long slivers of rock with a dioritic texture and composition. These may be several metres in length and many times longer than they are wide with their long axes lying in the plane of the foliation in the quartz diorite. Another variety of enclave is particularly common occurring as dark, fine-grained mafic pods with an amphibolitic composition. These are mostly quite small, usually no more than 300mm in diameter and have rounded, sometimes lobate, boundaries. They are of distinctly different appearance from the rafts of banded country rock and have probably been brought up from a lower crustal level.

The Northern granitic sheet, whilst possessing clear textural affinities to the Little Sark sheet, is slightly more acid in composition varying from quartz diorite to granodiorite. Like the Little Sark sheet it contains an interesting variety of enclaves. Recognisable rafts of country rock are only common near the contact (e.g. in the track at 46257700). Dioritic xenoliths are locally common, a prominent belt of these slivers runs down the N.W. side of La Grune (45877790). The xenoliths display all stages of assimilation, the surrounding granite often containing residual clots of biotite and hornblende.

A number of general field observations may be made that are relevant to the origin of the foliation in all the intrusive sheets. Firstly, the granitic fabric mimics but is younger than the gneissic foliation of the envelope. Secondly, the granitic fabric is often strongly penetrative. particularly in the Creux Harbour- Port du Moulin sheet where it sometimes penetrates gneissic xenoliths. This has resulted in the use of the term "Creux Harbour Gneiss" by some previous workers (e.g. Sutton and Watson 1957). Thirdly, the granitic foliation is cut by unfoliated granitic pegmatites (e.g. N.W. La Grune). Finally, in the Northern Sheet the foliation locally has been folded about south-facing asymmetric folds and is cut by north-dipping zones of cataclasis.

Although the intensity of the granitic fabric suggests a strong regional deformation, no sign of this has been detected in the gneisses. The cross-cutting granitic pegmatite veins argue for fabric development during the waning stages of melt solidification and it is suggested that the stress field acting on the granitic sheets was controlled by the geometry of the pre-existing gneissic envelope with the deformation apparently localised within the granitic sheets.

The intrusive sheets are often sheared along the foliation and are cut by many late high angle faults, some of which show signs of mineralisation. Silver, lead and copper were mined on Little Sark during the early 19th century and although the venture ultimately proved a financial failure considerable quantities of ore were obtained (Gibbons 1975).

The regular jointing in the granitic sheets has been exploited by a variety of dykes including several types of dolerite and later E-W striking iamprophyres and felsites. Petrography Wooldridge (1925) described the petrography of the Sark granitic rocks and noted their close affinities to tonalites. He was struck by the abundant field evidence for contamination of certain of the granites by country rocks. He emphasised the breakdown of hornblende to biotite and traced stages of this reaction between both xenoliths and hornblende xenocrysts and granitic magma. He also suggested that rocks composed of equal

amounts of hornblende and oligoclase are not a product of normal diorites and owed their origin to assimilation. Examination of a series of thin sections representative of all the Sark granitic sheets confirms that they vary in composition from quartz diorite to granite. They are composed of varying proportions of plagioclase, hornblende, biotite and quartz with K-feldspar sometimes locally becoming quite prominent. The Little Sark sheet is composed entirely of quartz diorite except for the granite already mentioned close to its northern contact. The sheets at Creux Harbour and Port du Moulin contain more hornblende and biotite close to their contacts and are more felsic away from the contacts. The Northern Sark sheet is variable in composition but quite often contains relatively abundant K-feldspar. Plagioclase is typically sodic andesine in the quartz diorites but is albitic in the granodiorites. However, zoning, sometimes interpreted as indicating an igneous origin, is rarely observed in the plagioclases. Hornblende may occur in a range of habits from euhedral to quite irregular ragged grains. Some of the more acid rocks, for example the Grande Gréve granite, contain no hornblende. Biotite is a common constituent and although often associated with hornblende more usually appears to be in textural equilibrium with it rather than forming as a breakdown product from it. K-feldspar always shows microcline twinning and may be rimmed by myrmekitic intergrowths. Accessory minerals include apatite, sphene, opaque minerals and zircon and the Northern granitic sheet contains rare euhedral zoned crystals of allanite. Hornblende and biotite with a strong preferred orientation often define a marked foliation in the rock. This suggests that the foliation was formed at a relatively high temperature. In a few specimens a distinctly different fabric was observed where slightly augenshaped plagioclases with chlorite developed along grain boundaries are surrounded by bands of recrystallised quartz. There is also evidence of late stage cataclasis in some specimens with micro-faulting and internal straining of K-feldspar, complex recrystallisation of plagioclase and quartz and a new growth of actinolite on hornblende, but these are of restricted occurrence. There are a number of important modifications superimposed on the mineralogy of the granitic rocks. Plagioclase is often extensively altered to a cloudy mass of fine-grained micaceous and opaque minerals but epidote group minerals are uncommon alteration products within the plagioclase. Some plagioclase shows clear patches and rims suggesting recrystallisation to albite in these areas. Chlorite may become an important constituent of the rock. Not all the chlorite seems to have developed as an alteration product of hornblende although, hornblende pseudomorphs and hornblende with reaction-rims of chlorite and sphene do occur.

Prehnite is locally abundant in some rocks and is found growing along both biotite and chlorite cleavages. Late stage cracks may be infilled with calcite or zoisite and clinozoisite particularly in specimens collected near contacts with amphibolites. All these features suggest either late stage deuteric alteration or metamorphism and local metasomatic movement. No pattern to the development of these effects could be defined from our sampling although there was some suggestion that they were more common to the west and south of the island. That the rocks have a complex thermal history is apparent from the range of modifications and textures observed. Some examples of chlorite growing from hornblende actually showed evidence for a new growth of biotite within the chlorite. It seems likely that many of these late alteration reactions were governed by very localised variations in chemical conditions. Chemistry Thirty-six specimens from the three main groups of granitic rocks and two specimens from enclaves have been chemically analysed by X-ray fluorescence using the methods of Brown and others (1973). The major elements values used have been recalculated to 100% on a water-free basis. The analyses have been plotted on a series of SiO2 variation diagrams as a convenient means of displaying and comparing the data. The granites show all the characteristics ascribed to a calc-alkaline series. The major element plots, with the exception of the alkalis (Figure 2), show strong correlation with SiO2 content. Each group of samples generally clusters on its own part of the trend. The Little Sark specimens are clearly more basic than the rest. Those from Central and Northern Sark show a similar range of composition. The Grand Gréve samples have the most acid compositions. Two specimens of the small dark fine-grained rounded enclaves from the Little Sark quartz diorite were analysed and they are plotted as open circles on the diagrams. They were the most basic rocks analysed and are olivine normative. It was considered that these might represent restite, the Grand Grave rocks separated melt and the other Little Sark specimens varying proportions of restite and melt. In a simple closed system model all these compositions should plot on a straight line on a SiO2 variation diagram. This does appear to be true for many of the elements plotted but important exceptions to this are Al2O3, TiO2 and P2O5 and also the trace element, Zr. Thus a simple model of the mafic enclaves as restite is untenable but further investigation of the chemical composition of a larger sample of the enclaves might prove rewarding. The plots provide convincing evidence that the granitic rocks have been contaminated by reaction with country

Figure 2. Plots of Major element oxides (in weight 47) against SiO2 and various other chemical plots for the foliated granitic rocks of Sark. Key to symbols used on Figures 2, 3 and 4: Filled circles; Little Sark; Open circles: Enclaves from Little Sark; Open squares: Central Sark sheets: Open diamonds: Central Sark sheets emplaced into amphibolites, Filled stars: Northern Sark sheet.

Figure 3. Plots of Ni. Cr, Rb and Sr (in parts per million) against SiO2 and various other chemical plots for the foliated granitic rocks of Sark. See caption to Figure 2 for key to symbols.

Figure 4. Plots of Ba, La, Ce, Nd, Y, Sc, V, Zn, Zr, Nb (in parts per million) against Si02 for the foliated granitic rocks of Sark. See caption to Figure 2 for key to symbols.

rock. The specimens considered most likely to show contamination are those from Hogshack and the west coast faulted sheets full of amphibolitic enclaves, all of which are plotted on the diagrams as diamonds. On the major element plots (Figure 2) the contaminated specimens clearly contain relatively higher MgO and probably lower alkalis as would be expected as a result of contamination by amphibolite. Other major elements do not Show such clear cut evidence. However, the most striking evidence of amphibolite contamination is seen in the marked high relative Cr and Ni contents (Figure 3) of the Hogsback type sheets. The more basic parts of the Creux Harbour and Port du Moulin sheets occur closer to amphibolitic gneiss and they might be expected to he contaminated. However, although they have higher Ni and Cr contents than the more acid parts of the sheets; the values are no higher than those of the Little Sark samples and no discrimination is detectable on the major element plots. There is no clear-cut evidence that the more basic nature of these sheets near the contacts is a result of contamination. The trace elements Ba, La, Ce, Nd and Y all show considerable scatter when plotted against SiO2 (Figure 4) although there may be progressively less scatter shown from light to heavier rare earth elements. The more acid rocks tend to have higher La/Y values (Figure 3) which suggests a relative enrichment of light rare earth elements in these rocks. Ba occupies the same sites as K in crystal lattices and a plot of Ba against K (Figure 3) shows reasonable correlation. This suggests that the same factor that leads to the scatter in the alkali element plots, probably the secondary mineralogical modification of the rocks, also affected the distribution of Ba. The scatter for La, Ce, Nd and Y could also be due to secondary processes. The presence of hydrothermal fluids is strongly suggested by the formation of Prehnite. The fluids could have erratically redistributed rare earth elements released from recrystallising hornblende and sheet silicates to produce the present distribution. The trace elements Sc, V, Zn, Zr, Nb also show good inverse correlations with SiO2 content (Figure 4) generally having their highest values in the Little Sark rocks. There seems little evidence for secondary redistribution of these elements although the Grand Gréve rocks do appear to have anomalously high Zn values (and Pb values, not shown on the diagrams). Rb and Sr show rather erratic distributions and Rb/Sr values (Figure 3) have a restricted range suggesting that rocks of this type are not very suitable for Rb-Sr age determinations. Any future .attempt to obtain a more precise Rb-Sr whole rock isochron for the rocks would do well to avoid specimens that might be contaminated by the older gneisses.

Final statement There seems to be little evidence, either field or chemical, to suggest that the foliated calc-alkaline granitic sheets of Little Sark, Central Sark and Northern Sark are other than of the same age and related origin. There is petrographical and geochemical evidence for local contamination of the intrusions by the country rock. The abundance of country rock xenoliths showing all stages of assimilation near the contacts suggests slow cooling at relatively deep crustal levels. The secondary mineralogical modifications superimposed upon the original igneous texture may also be a result of a complex cooling history although this could reflect a later metamorphism. The granitic foliation, cut by pegmatitic veins, is tentatively ascribed to local stresses operating upon the sheets during, or soon after, intrusion and final consolidation. Information on igneous activity during the early phases of the Cadomian orogeny is rather sparse and indefinite. Although foliated quartz dioritic intrusions, petrographically similar to the Sark sheets are found elsewhere in the Channel Islands (Perelle gneiss on Guernsey; Westerly quartz diorite on Alderney) much older dates have been suggested for these (Adams 1976; Roach 1977). Detailed petrographic and geochemical studies followed by a related programme of age determinations would provide a sounder basis for correlation and reconstruction of Precambrian events in this region. Acknowledgements. We are grateful to Dr D.J. Hughes for constructive discussions in the field. References Adams, C.J.D., 1976. Geochronology of the Channel Islands

and adjacent French mainland. J. Geol. Soc. Lond., 132, 233-250.

Brown, G.C., Hughes, D.J., and Esson, J., 1973. New X.R.F. data retrieval techniques and their application to U.S.G.S. standard rocks. Chem. Geol., 11,223-229.

Gibbons, W., 1975. Rocks of Sark. 75pp. Manche Technical services, Jersey.

Gibbons, W. and Power, G.M., 1975. The structure and age of the gneisses of Sark, Channel Isles. Proc. Ussher Soc., 3, 244-251.

Roach, R.A., 1977. A review of the Precambrian rocks of the British Variscides and their relationships, with the Precambrian of N.W. France. In: 'La chaine varisque d'Europe moyenne et occidentale - Coll. intern, C.N.R.S., Rennes, 243, 61-79.

Sutton, J. and Watson, J., 1957. The structure of Sark, Channel slands. Proc. Geol. Assoc.; 68, 179-204.

Wooldridge, S.W., 1925. The petrology of Sark. Geol. Mag., 2, 241-252.

Written discussion Dr P.A. Floyd: (a) From the development of trends in the FMA diagram and on the various element plots against SiO2, are you suggesting that all the rocks from quartz diorite to granite are related via fractional crystallization? (b) SiO2 may not be a good differentiation index as it is possible that it might have been mobile during low-grade alteration and thus contribute to the scatter seen in some of the trace element plots, especially where "immobile" elements are used. Authors' reply: (a) From the similarity of the field relationships and the trends formed on the geochemical diagrams we are suggesting that the rocks have a related origin, but we are unable to say that this is the result of fractional crystallisation. (b) Dr Floyd raises a very interesting point in suggesting the mobility of SiO2 during the low grade alteration, and this must be likely. However, SiO2 shows good inverse correlations with CaO, MgO, TiO2, FeO, P2O5 and some trace elements without any scatter. We would, therefore, prefer to consider the scatter produced on some "immobile" element plots as the result of movement of these so-called "immobile" elements during the low grade alteration.

Read at the Annual Conference of the Ussher Society, January 1980 Primary and secondary chemical variation exhibited by some west Cornish volcanic rocks P.A. FLOYD A.M. AL-SAMMAN

Volcanic associations Four main geographical groupings of volcanic rocks occur in west Cornwall: (a) a linear belt of associated intrusive sills and lavas southwest of Redruth-Camborne (Camborne group) (b) a single (?) pillow lava horizon with closely associated intrusives in the north of the Penwith peninsula from St Ives to Kenidjack Castle (Clodgy group), (c) a broad zone of separate, thick intrusive sills and lavas, some pillowed, around Penzance (Penzance group) and (d) various massive intrusive sills on the south Cornish coast between Marazion and Porthleyen (Cudden-Perranuthnoe group). Horizons of very fine-grained massive bodies found within the contact aureoles of the Land's End and Carnmenellis granites are interpreted as lava flows. Some lavas which exhibit recognisable pillows are clearly submarine in origin and it is possible that intense s hearing and subsequent hornfelsing may have obscured other pillow lava horizons to produce apparently uniform "lava" bodies. Massive, medium, to coarse-grained doleritic intrusives tend to retain their identity and in contact aureoles may still contain original clinopyroxene at low grades.

In any one volcanic group there is only a general association of intrusives and lavas as they may be separately developed in any one location. The intimate relationship of high-level sills intrusive into pillow lava sequences tends to be the exception and many massive bodies are intrusive into sediments generally devoid of submarine lavas (e.g. the south coast group). The age of the submarine lavas and the enclosing Mylor sediments are generally considered to be Lower Devonian (Dearman 1971, fig. I; Wilson and Taylor 1976, fig. 5), whereas many of the intrusives, which often post-date the main phase of deformation (Fl) in west Cornwall (Taylor and Wilson 1975) could have been emplaced later in the Devonian. Based on palaeontological evidence from a single locality at Mount Wellington, Turner and others (I 974) have suggested that the Mylor series are Upper Devonian and equate the enclosed pillow lavas with those in the Frasnian slates of N. Cornwall at Pentire Point.

P.A. Floyd and A.H. AI-Samman 1980. Primary and secondary chemical variation exhibited by some west Cornish volcanic rocks. Proc. Ussher Soc., 5, 68-75. The spatially associated intrusive (dolerite sills) and extrusive (submarine lavas and pillow lavas) volcanics of west Cornwall suffered chemical and mineralogical change during low-grade regional and contact metamorphism. On the basis of the immobile elements (Ti, P, Zr, Y, Nb, La and Ce) the different volcanic groups at Redruth-Camborne (Camborne group), within the Penwith peninsula (Penzance group) and along the south coast (Cudden-Perranuthnoe Group) form three chemically separate, differentiated comagmatic basalt suites possibly related by varying degrees of partial melting. The intrusives and extrusives within each group may be directly related by either progressive fractionation (Camborne group) or exhibit the same degree of differentiation and chemical range (Penzance group). The Cudden-Perranuthnoe group are chemically the most primitive and contain olivine-pyroxene cumulates. The Camborne intrusives are characterized by high initial Y abundances which reflect either extensive partial melting of major Y-bearing mantle phases (garnet) or selective melting of minor phases (amphibole or apatite). The volcanics are tholeiitic basalts and representative of within-plate volcanism. Their composition supports the notion of mantle heterogeneity for S.W. England, with parental compositions lying on the partial melting curvetypical of west and south Cornwall. P.A. Floyd, Department of Geology, University of Keele, Staffordshire, ST5 5BG: A.Jt. A1-Samman, Department of Geology, College of Science, University of Mosul Mosul Iraq

Objectives and previous research Forty-one samples of representative intrusives and lavas were collected and chemically analysed (by XRF and wet chemical methods) from three main volcanic areas: Camborne, Penzance and part of the south coast adjacent to Cudden Point and Perranuthnoe (actual sampling localities are given by Al-Samman, 1980). The object was to determine the primary chemical variation and genetic relationship of the volcanics after removing the chemical effects of secondary alteration due to regional and contact metamorphism. In particular were the spatially associated intrusives and lavas simply related by crystal fractionation and did each volcanic area have its own chemical identity? Elsewhere in S.W. England trace element studies have demonstrated that each volcanic location is unique and typified by its own "primary" basalt composition and comagmatic series with a characteristic chemistry (Floyd and others, in press). These suites are dominantly alkaline in chemistry, although in west Cornwall (excluding the Lizard complex) some rocks with tholeiitic affinities are present, e.g. Cudden Point greenstone sill (Floyd and Lees 1973), Mullion Island pillow lavas (Floyd and others 1976) and the north Penwith peninsula pillow lavas (PAF, unpublished data). Analysis of the volcanics in the three sampling areas was undertaken to determine if west Cornwall generally was a tholeiitic province relative to the alkaline nature of Devonian volcanism elsewhere in S.W. England (Floyd 1976). Many workers have studied the petrology of the volcanics in the area (especially within the aureole of the Land's End granite), although relatively few chemical analyses have been published (Phillips 1876; Tilley 1935; Hawkes 1958; Floyd 1965; Floyd and Lees 1973; Parker 1970). None have considered the primary chemical relationship between the volcanic rocks. Petrological assemblages Initially the volcanics were affected by the regional metamorphism that accompanied early Variscan deformation and then some were subsequently thermally metamorphosed within the Land's End and Carnmenellis granite aureoles. The partial preservation of original textures, relict clinopyroxenes and the metamorphic assemblages developed all indicate that the lavas and pillow lavas were basalts and the intrusives (greenstones) were dolerites with minor gabbros. Petrographic details of the metabasic rocks and their hornfelsed equivalents have been given by Al-Samman (1980); the various common assemblages are summarized in Table 1. When seen outside the aureoles the extrusives have suffered greater mineralogical change due to regional metamorphism than the intrusives, such that no primary

minerals are left. The intrusives sometimes contain primary clinopyroxene (�pigeonite exsolution lamellae) that is invariably pseudomorphosed by fibrous actinolite. A characteristic feature of the lava hornfelses is the variable development of biotite and calc-silicate zones, especially within the hornblende hornfels facies of the aureoles. Table 1. Summary of meta-basic assemblages exhibited by the volcanics of west Cornwall. Intrusive Volcanics Extrusive Volcanics

Regional metamorphic environment

epidote-chlorite-albite-actinolite- epidote-chlorite clinopyroxene albite epidote-chlorite-albite-actinolite ± calcite-chlorite calcite albite chlorite-albite-actinolite ± stilpnomelane sericite-quartz chlorite-albite quartz-chlorite-albite-actinolite

chlorite-albite

Contact metamorphic environment chlorite-albite-actinolite-clinopyroxene albite-actinolite ± biotite albite-achnolite ± biotrte ± axinite andesine-biotite hornblende andesine-hornblende ± biotite andesine-diopside hornblende hornblende andesine-diopside grossularite hornblende Secondary chemical effects Secondary processes such as submarine weathering and low-grade metamorphism cause some elements to be mobile and thus change or mask the original composition of basaltic rocks. In west Cornwall the problem of element migration is compounded by contact metasomatism and remobilization superimposed on any regional effects. The nature of any secondary chemical change can be evaluated by (a) comparing rock groups showing progressive alteration (usually measured by the H20+ content), (b) the disruption of typical magmatic trends and element associations, and (c) by comparing rocks of different hornfelsic grade in the contact environment.

Regional hydration Both intrusives and extrusives have relatively low oxidation ratios (Fe203/Fe04<0.6) and in alteration terms are typified by varying degrees of hydration (H20+ from 1 - 6 wt. %). As a group the regional metamorphosed volcanics exhibit decreases in Ca and Sr, together with apparent Mg, Ni and Cr increases with progressive hydration. Typical magmatic coherence between Ca and Sr is completely lost, whereas between Mg-Ni and Mg-Cr it is still maintained and suggests that the high transition element contents are original and indicative of olivine-rich basaltic rocks. Complete replacement by hydrous Mg-bearing phases, such as chlorite and serpentine minerals would produce a correlation between Mg etc. and H20+ content. Of the alkali elements, only Na shows any suggestion of an increase with progressive hydration; K, Rb and Ba distributions are variable and non-systematic.

The regional metamorphosed rocks show chemical variation similar to that described for greenstones in S.W. England (Floyd 1976) and spilitic rocks in general (Melson and others 1968; Cann 1969). In mineralogical terms the alteration is reflected in the development of calcite-epidote-chlorite segregation domains, pseudomorphous replacement by chlorite and actinolite, and possibly calcite-chlorite vesicular infilling.

Contact metasomatism

The volcanics within the contact aureoles have been affected by two processes: (a) redistribution of Ca, Sr and Fe within the lavas with the development of calc-silicate segregations, and (b) addition of K and Rb from the adjacent granite and the development of biotite.

The first case reflects both initial regional segregation of secondary minerals together with local mobilization and redeposition of mainly Ca that on contact metamorphism produced irregular bodies of diopside-plagioclase-grossularite ± sphene ± epidote ± calcite. As seen in Fig. I the distribution of Ca and Sr in the volcanics is similar in both regional (greenschist facies) and low-grade contact (albite-epidote hornfels facies) environments. Volcanics representing higher grades of contact metamorphism (hornblende hornfels facies) have generally elevated Ca contents, that reflect variable mobility and subsequent segregation of Ca-rich, Sr-poor phases at higher temperatures

Metasomatism of the aureole rocks by K, Rb, Li, B and F is a typical feature of the S.W. England granites (Bowler 1958; Floyd 1965; Floyd and Fuge 1973) and many of the volcanics studied here have higher than usual K and Rb abundances (> 1 wt. % K20 and > 50 p.p.m. Rb) relative to typical basic rocks (Fig. 1). In view of the mobility of K and Rb in both the regional and contact environments they maintain good coherence (Fig. 1) with the contact rocks being relatively enriched in K and Rb. The high-grade hornfelses are preferentially enriched in Rb with an

average K/Rb ratio of 66 relative to 122 for all the lower grade rocks. The granites are also rich in the light rare earth elements (REE) (Lees and others 1978) and under contact conditions may have selectively enriched the aureole rocks and changed the normal distribution patterns between pairs of REE. Irrespective of metamorphic grade La and Ce show coherence at low concentration levels (Fig. I), although all the volcanics from the Penzance section of the Land's End aureole fall away from the main trend displayed and are enriched in La only. Treating Y as a heavy REE nearly all the same group of volcanics, but specifically the pillow lavas from Gulval (1.5 km N.E. of Penzance), are marginally depleted in Y relative to Ce such that normal coherence is lost for this group. Both La and Y have thus been selectively mobilized in these particular pillow lavas and possibly during the contact metamorphic development of diopside after hornblende. Primary chemical variation The primary composition of the volcanics has been masked by selective element mobility during regional and contact metamorphism. Elements that are essentially immobile (P, Ti, Zr, Y, Nb and REE) during alteration can be used to indicate magma type and the existence of different comagmatic suites, whereas the distribution of the transition trace elements (Ni, Cr) provide evidence for the involvement of mafic minerals during fractional crystallization. Magma type Using the Nb/Y - Zr/P205 plot (Floyd and Winchester 1975) that discriminates between alkaline and tholeiitic basalts, many of the volcanics are clearly tholeiitic, whereas others fall in the overlap zone between the two magma types (Fig. 2). These last rocks may be transitional in incompatible element terms, although the separate suites illustrated here have tholeiitic characteristics (constant Nb/Y with progressively increasing Zr/P205). Only the Gulval pillow lava suite of the Penzance extrusive group appears to be alkaline and distinct from the rest, although selective Y loss (as indicated above) would artificially increase the Nb/Y ratio.

Figure 1. Distribution of some elements that were mobile within or added to the granite aureoles during contact metamorphism of volcanics from west Cornwall.

Figure 2. The predominantly tholeiitic nature of various volcanic suites from west Cornwall. The three extrusives with apparently alkaline characteristics from Penzance are from the Gulval pillow lava group. Magma type designation from Floyd and Winchester (1975). Differentiated comagmatic suites Fresh basalts that are genetically related within a rock suite generally show good coherence between pairs of incompatible elements. If the element pairs produce divergent multiple trends in any one plot then a number of comagmatic suites are present, each possibly represented by a "primary magma" from which the rest of the suite may have evolved via fractional crystallization, The ratio of the incompatible elements (which for altered rocks are. represented by the immobile elements) defines or characterises each separate suite. Using the FeO*/MgO ratio (where FeO* = all Fe as FeO) as a rough differentiation index and various immobile element plots relative to Zr (Fig. 3) a number of chemically distinct volcanic suites are present in W. Cornwall. (a) The various intrusive bodies from the south coast (the Cudden-Perranuthnoe group) form a comagmatic series related via mafic phase fractionation as seen by the rapid decrease in Ni and Cr with increasing differentiation

index. The Cudden Point tholeiite contains the most primitive basaltic composition (excluding the cumulates with very high Mg, Ni and Cr) to which the other intrusives along the south coast are related. (b) The Penzance intrusives also show a trend of decreasing Ni and Cr with differentiation, although they appear to be chemically unrelated to the spatially associated lavas and pillow lavas. The separate grouping of the extrusives could, however, reflect their greater alteration which has caused a change in the FeO*/MgO ratio towards higher values. As seen in the plots against Zr (Fig. 3) the Penzance intrusives and extrusives are chemically very similar and cover essentially the same abundance range. However, there is a suggestion of two slightly divergent series with the extrusives being marginally higher in Nb, P, and lower in Ti, Y, than the intrusives at specific Zr contents. The best separation of the two groups is given by Nb versus Zr (Fig. 3) and the Nb/Y ratio (Fig. 2)

Figure 3. Distribution of Ni and Cr relative to FeO*/ MgO (used as a differentiation index) and correlation between various immobile incompatible elements in volcanics from west Cornwall. Field encloses the intrusives from the south coast Cudden-Perranuthnoe group; the broken line indicates that only a few samples plot in this section of the field. Dots and circles are intrusives and extrusives respectively from the Penzance area; solid and open triangles are intrusives and extrusives respectively from the Camborne area.

(c) In marked contrast to the majority of the Penzance extrusives, the pillow lavas at Gulval define a separate grouping and do not readily form part of any other trend (suite) developed. There is insufficient data to state whether or not the Gulval lavas are a different chemical suite. For example, their particular chemistry could reflect (i) the presence of cumulate ilmenite (producing high Ti and Nb), and (ii) the extensive replacement of hornblende by diopside during contact metamorphism (causing Y loss and La enrichment). (d) The Camborne intrusives also define a mafic fractionation trend and are separated from the Camborne lavas in a similar manner to the Penzance volcanics (Fig. 3). However, immobile element plots indicate that the Camborne group all fall on a single trend with the extrusives exhibiting more fractionated abundances than the intrusives. The intrusives display one unusual characteristic in having very high Y contents ( >30 p.p.m.) coupled with low "primitive" abundances of other incompatible elements; being in the latter respect similar to some of the Cudden Point intrusives. Discussion and conclusions On the basis of immobile element abundances and ratios the volcanic rock of west Cornwall are predominantly tholeiitic basalts that suffered chemical and mineralogical change during subsequent regional and contact metamorphism. There are a number of chemically distinct suites present, each characterised by. different incompatible element ratios and trends. The rocks in each suite are related principally via mafic fractionation involving olivine and pyroxene. The Camborne lavas appear to be related to the spatially associated intrusives and are more chemically evolved or fractionated. This is a commonly observed chemical relationship between intrusives and extrusives with the latter tapping the more fractionated t6p of a magma chamber, while the later intrusives are fed by lower,, less fractionated, levels as the chamber drains. On the other hand, the Penzance extrusives and intrusives exhibit two slightly, different, but overlapping fractionation trends. If derived from a single fractionating magma chamber variably fractionated melts from different parts of the chamber continuously reached the surface (as lavas) or were trapped below it (as intrusive sills). The different location and almost mutual exclusion of the intrusives relative to the extrusives at Penzance, however, suggests a more likely model would be of two closely associated (interconnected?) magma chambers containing melts with marginally different chemistries.

In terms of tectonic environment the majority of the west Cornish volcanics plot in the within-plate basalt field of the Ti-Y-Zr diagram of Pearce and Cann (1973). Most of the suites have Zr/Y ratios greater or slightly less than 4; values which again typify within-plate basalts (Pearce and Norry, 1979). The only group that markedly differs from this designation are the high-Y Camborne intrusives that fall in the low-K tholeiite field and have an average Zr/Y of ~2. The overall high Y content is unusual relative to the other volcanics and either reflects (i) a high degree of partial melting whereby Y is released from a major host mineral in the mantle such as garnet, or (ii) lower degrees of melting with the selective fusion of minor mantle phases like apatite or amphibole. Floyd and others (in press) have suggested that the primary chemical variation exhibited by the S.W. England Upper Palaeozoic volcanics can, in part, be attributed to mantle source heterogeneity. The volcanics studied here support this suggestion and could be derived by varying degrees of partial melting from the' same mantle composition as other magmatic rocks in west and south Cornwall and are distinct from those of north Cornwall and Devon. Acknowledgements. A.H. AI-Samman acknowledges the receipt of an Iraqi Government research grant to undertake an M.Sc degree, of which this work forms a part. P.A. Floyd is grateful to the University of Keele Research Fund for a grant to cover field work expenses. G.J. Lees and D. Emley are thanked for considerable assistance and advice on chemical analysis .

References Al-Samman, A.H. 1980. Petrology and geochemistry of some

volcanic rocks from S. C. Cornwall, S.W. England Unpubl. M. Sc. thesis, Univ. of Keele.

Bowler, C.M.L. 1958. Distribution of alkalis and fluorine across some granite-killas and granite-greenstone contacts. Abs.Proc. Conf. Geol. & Geomorph., S. W. England, Exeter, 20-21

Cann, J.R. 1969. Spilites from the Carlsberg Ridge, Indian Ocean. J. Petrology, 10, 1-19.

Dearman, W.R. 1971. A general view of the structure of S.W. England. Proc. Ussher Soc., 2, 220-236.

Floyd, P.A. 1965. Metasomatic hornfelses of the Land's End aureole at Tater-du, Cornwall. J. Petrology, 6, 223-245.

Floyd, P.A. and Fuge, R. 1973. Distribution of F and CI in some contact and regionally metamorphosed Cornish greenstones. Proc. Ussher Soc., 2, 483-488.

Floyd, P.A. and Lees, G.J. 1973. Ti-Zr characterization of some Cornish pillow lavas. Proc. Ussher Soc., 2, 489-494.

Floyd, P.A. and Winchester, J.A. 1975. Magma, type and tectonic setting discrimination using immobile elements. Earth planet. Sci. letters, 27, 211-218.

Floyd, P.A. 1976. Geochemical variation in the greenstones of S.W. England. J. Petrology, 17, 522-545.

Floyd, P.A., Lees, G.J. and Parker, A. 1976. A preliminary geochemical twist to the Lizard's new tale. Proc. Ussher Soc., 3, 414425.

Floyd, P.A., Exley, C.S. and Stone, M. 1980. Composition and petrogenesis of the magmatic rocks of S.W. England (in press)

Hawkes, J.R. 1958. Petrological features of the greenstones and sediments in the Carn Moyle to St. Ives section of the Land's End aureole. Abs. Proc. Conf. Geol. & Geomorph.,S.W. England, Exeter, 12-14.

Lees, G.J., Alderton, D.H.M., Pearce, J.A. and Exley, C.S. 1978. Rare earth elements in acid rocks of S.W. England (Abstract). Proc. Ussher Soc., 4, 217.

Me[son, W.G., Thompson, G. and Van Andel, T. 1968. Volcanism and metamorphism in the Mid-Atlantic Ridge, 220N latitude. J. geophys. Res., 73, 5925-5941.

Parker, A. 1970. Chemical and mineralogical analyses of some basic and ultrabasic rocks and their initial weathering products. Reading Univ. Geological Rep. no. 4, 1-44.

Pearce, J.A. and Cann, J.R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth. plant. Sci. letters, 19, 290-300.

Pearce, J.A. and Norry, M.J. 1979. Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks. Contrib. Mineral. Petrol., 69, 33-47.

Phillips, J.A. 1876. On the so-called greenstones of W. Cornwall Quart. J. geol. Soc., Lond., 32, 155-179.

Taylor, R.T. and Wilson, A.C. 1975. Notes on some igneous rocks of west Cornwall. Proc. Ussher Soc., 3, 255-262.

Tilley, C.E. 1935. Metasomatism associated with the green-stone-hornfelses of Kenidjack and Botallack, Cornwall. Mineralog. Mag., 24, 181-202.

Turner, R.E., Taylor, R.T., Goode, A.J.J. and Owens, B. 1979. Palynological evidence for the age of the Mylor slates, Mount Wellington, Cornwall. Proc. Ussher Soc., 4, 274-283.

Wilson, A.C. and Taylor, R.T. 1976. Stratigraphy and sedimen-tation in West Cornwall. Trans. R. geol. Soc. Cornwall., 10, (1971-1974), 246-259.

Read at the Annual Conference of the Ussher Society, January 1980

The distribution of lithium in the St Austell Granite J. DANGERFIELD J.R. HAWKES E.C. HUNT

J. Dangerfield. J.R. Hawkes and E.C. Hunt 1980. Thee distribution of lithium in the St Austell Granite. Proc. Ussher Soc., 5, 76-80. Lithium values for British igneous rocks are contrasted with newly determined figures for the St Austell Granite, Cornwall. Within this pluton, the element shows marked enrichment not only in a late lithium-mica granite phase, but also in adjacent biotite-granite. Hydrous magmatic alteration of the lithium-mica granite resulted locally in lithium depletion. However, subsequent kaolinisation processes apparently caused no significant redistribution or depletion of the element. J. Dangerfield and J.R. Hawkes, Institute of Geological Sciences, Exhibition Road, London S W7 2DE; E.C. Hunt, Laboratory of the Government Chemist Unit, National Physical Laboratory, Teddington, Middlesex TWl l OL W

Introduction The amounts of lithium contained in various rock types are not particularly well documented, and quoted mean figures tend to be ambiguous because they rest on too few analyses. Thus in the Handbook of Geochemistry, Heier and Billings (1969) presented several mean values for granitic rocks ranging from 20 ppm Li to 40 ppm Li. A compilation based on data from British acid igneous rocks (Beer and others 1978, table 4, p.284) is shown in Table 1. Table 1 number of mean samples (ppm Li)

18 granodiorites 33 88 unclassified granites (probably

mainly granodiorites) 26 145 granites 288 47 acid volcanic rocks (all types) 32

For comparison, a weighted mean value of 24.6 ppm Li can be derived for all other major igneous rock groups in Britain using information given by Beer and others (1978, table 1, p.282). Although these figures should be regarded as approximate, it seems that the quantities of lithium present in granites (Streckeisen 1976) may be greater than those in other common igneous materials by a whole order of magnitude. However, most of the analyses used

in formulating the mean granite value of 288 ppm Li are of samples from the South-West England plutons. The figure is therefore largely a reflexion of the lithium tenor of such rocks in this particular province. From the sources quoted in Beer and others (1978), whole-rock lithium data relating specifically to the Variscan granites can be tabulated as shown in Table 2. Table 2 number of mean & range samples (ppm Li)

6 lithium-mica granite (Tregonning- 1119 (185-2276) 7 Godolphin Pluton) lithium-mica granite (Meldon 4229 (1951-7400)

104 Aplite Dyke) biotite-granite (Dartmoor, Bodmin, St Austell, Carnmenellis, Land's End & Kit Hill plutons) 272 (40-600)

11 partly greisenized, biotite-granite 703 (340-1400) (Cligga Head Pluton)

Except for material from the Cligga intrusion, which is a local case requiring further investigation, the rocks fall into two obvious groups: those containing lithium-mica ("lithionite") and those characterised by biotite-mica. Both rock types occur in the St Austell Granite and analyses of 281 samples from the mass form the basis of this paper.

Figure 1. The distribution of rock types in the St Austell Granite.

The main features of the St Austell Granite The pluton comprises three distinctive rock types which crop out as indicated in Figure 1. 1. Coarse-grained biotite-granite containing feldspar megacrysts which vary in proportion from scattered individuals to local concentrations forming up to 65 per cent of the rock by volume. In general, a range from I to about 30 per cent is observed. The distinction made on the map between megacrystic and poorly megacrystic granite is at an arbitrary feldspar megacryst content of 10 per cent. 2. Fine-grained granite, generally containing abundant megacrysts of feldspar, quartz and biotite. 3. Medium-grained, lithium-mica granite devoid of megacrysts and locally showing alteration to "gilbertite"-and fluorite-bearing material. Field relationships suggest that the fine-grained granite represents material included within the coarse-grained megacrystic phase. It occurs as large, irregular masses, (see Fig. 1), also sheet-like bodies interlayered with coarse-grained granite, and as rounded pods commonly only a few centimetres across. Brecciation and veining by coarse-grained granite can be seen in Gaverigan China Clay Pit (SW.925 581). The lithium-mica granite appears to be intrusive into both the coarse-grained and fine-grained varieties, although by analogy with similar rock developed in the Tregonning-Godolphin Pluton, a metasomatic origin cannot be ruled out (Stone 1975). The localised introduction of fluorine and the alteration of lithium-mica to "gilbertite (greenish muscovite) were probably effected by late-stage hydrous magmatic fluids. After a lapse of 15-20 my, the whole region suffered a period of volcanism, which resulted in the intrusion of quartz-porphyry (elvan) dykes and attendant development of quartz and quartz-tourmaline veins, some also carrying metalliferous minerals (Hawkes 1974). A further stage in this activity led to the partial destruction of plagioclase within large areas of the granite pluton (Exley 1959). All rock types, inc!uding the elvan dykes, were affected. These regions are characterised now by kaolin, possibly generated much later by relatively low-temperature weathering processes during Cretaceous or Tertiary times (Sheppard 1977). Method of analysis and results Bulk specimens of 2-3kg were crushed and sampled conventionally to produce about 100g of fine powder. A lg sample of this was decomposed with hydrofluoric, nitric and perchloric acids in a PTFE pressure digestion vessel. Removal of hydrofluoric acid by evaporation preceded the addition of hydrochloric and boric acids and the preparation of a stock solution bY dilution with water.

A portion of this solution was diluted with the addition of potassium ions and the lithium content determined by flame emission spectrometry against standards prepared similarly in a synthetic granite solution. The results of analysis are shown in Table 3 Table 3

The mean and range figures for the whole outcrop of coarse-grained granite are much higher than those derived for the Variscan plutons in general (Table 2)., due mainly to 20 samples, collected near outcrops of lithium-mica granite, which contain more than 1000 ppm Li. Excluding all samples with more than 600 ppm Li (the highest value recorded for coarse-grained biotite-types elsewhere in the province- see Table 2), the remaining 102 specimens yield a mean figure of 314 ppm Li. The lithium content of fine-grained granites (472 ppm) is also higher than might be expected, due again to a few samples from exposures adjacent to the lithium-mica granite with more than 1000 ppm Li. The results clearly show that "gilbertisation" markedly reduced lithium levels in the lithium-mica granite, 17 completely altered samples providing a mean value of only 50 ppm Li, compared with the general figure of 1615ppm Li for fresh and partly affected specimens. Conversely, it seems that kaolinisation had no such systematic effect on the lithium content of any rock-type in the pluton. In many cases, unkaolinised and fully kaolinised materials from the same locality yield comparable results; in others kaolinised specimens may contain either more or less lithium than the neighbouring unaltered rocks. The distribution of lithium Figure 2 shows the distribution of lithium values obtained from specimens within the St Austell Pluton. Because sample localities are relatively widely spaced the boundaries between variously shaded areas represent no more than generalised contours of whole-rock lithium content. Comparison with Figure 1 demonstrates a predictably clear correlation between the occurrence of high lithium values and outcrops of lithium-mica granite. Perhaps less expected is the shape of the area of relative lithium enrichment in coarse-grained granite adjacent to lithium-mica granite. This area does not exactly coincide

number of mean & range samples (ppm Li)

57 Lithium-mica granites 1615 (200-3060) 17 Lithium-mica granites 50 (15-90)

“gilbertised” 171 Coarse-grained biotite-granites, 558 (75-2650)

megacrystic & poorly megacrystic 36 Fine-grained granites 472 (30-1630)

Figure 2. The distribution of lithium in the St Austell Granite.

with the area designated porphyritic lithionite-granite by Exley (1959), nor does it include parts of the intrusion where fine-grained granite predominates over the coarse-grained variety. The extreme western lobe of the pluton, consisting both of coarse-grained and fine-grained granite varieties, is also excluded from the area of relatively high lithium even though it directly adjoins the main outcrop of the lithium-mica granite. Conclusions The apparent distribution of lithium leads to the following suggestions:- 1. Because amounts of, the element in the eastern and extreme western megacrystic granite (approximate mean 314 ppm Li) are comparable with those of the other major Variscan plutons, the enhanced values for poorly megacrystic material occupying the west-central part of the intrusion probably denote the presence of an irregular metasomatic aureole associated with the lithium-mica granite. Field observation shows that biotite in these regions has certainly suffered partial alteration to a pale-brown lithian species or series of species. 2. From evidence contained in the first suggestion, il appears likely that lithium-mica granite occurs at no great depth throughout the entire west-central region. 3. Generally lower lithium values in the fine granitic rocks could be due to a relative imperviousness to the proposed metasomatising fluids. 4. The low levels of lithium in the western lobe of the mass suggest that this portion may have a faulted relationship with the adjacent lithium-mica granite area. 5. Removal of lithium during "gilbertisation" contrasts with only partial disturbance of the element during the widespread destruction of plagioclase in the granite and eventual development of kaolinite. This points to significant differences in the physical and chemical nature of the fluids involved. "Gilbertisation" appearsto have been a late magmatic event: kaolinisation may have been a two-stage process (Bristow 1977) connected initially with a late hot spring phase of the elvan volcanic episode and, subsequently, with .weathering. Acknowledgements are due to landowners, particularly English Clays Lovering Pochin and Co. Ltd. and the Goonvean and Rostowrack China Clay Co. Ltd.. for allowing access to their properties. Thanks are also due to Miss S.K. Pepper and Mr D.J. Rodda for analytical assistance. Mr R.K. Harrison and colleagues in the S.W. England Field Unit kindly criticised the manuscript. References Beer. K.E., Edmunds. W.M. and Hawkes, J.R. 1978. A pre-

liminary look at lithium in the United Kingdom. Energy, 3, 281-292.

Bristow, C.M. 1977. A review of the evidence for the origin of the kaolin deposits in S.W. England. Proc. 8th Int. Kaolin Symposium and Meeting on Alunite, Madrid-Rome, Num. K-2, 19pp.

Exley, C.S. 1959. Magmatic differentiation and alteration in the St Austell granite. Q. Jl. geol. Soc. Lond., 114, 197-230.

Hawkes, .I.R. 1974. Volcanism arid metallogenesis: the tin province of South-West England. Bull. Volcanologique, 38, 1125-1146.

Heier, K.S. and Billings, G.K. 1969. Lithium. In Handbook of Geochemistry, 11-I, 3.E. 1.-3, E.4. Springer-Verlag, Berlin.

Sheppard, S.M.F. 1977. The Cornubian batholith, S.W. England: D/H and 180/160 studies of kaolinite and other alteration minerals. Jl. geol. Soc. Lond., 133, 573-591.

Stone, M. 1975. Structure and petrology of the Tregonning- Godolphin granite, Cornwall. Proc. Geol. Assoc., 86, 155-170

Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Sci. Rev., 12, 1-33.

This paper is published by permission of the Director, Institute of Geological Sciences. Written discussion Dr C.S. Exley: I should like to make two comments and ask two questions. First, it is not surprising, nor need it cause concern, that the eastern junction between the third lowest and fourth lowest concentration zones of lithium does not correspond exactly with earlier junctions based on texture or composition. Despite the extension of clay workings, exposure remains poor right across that strip of the granite, and variation in specimen collection will produce variation in the shape and position of the junction. Secondly, my distinction between biotite and 'lithionite' (which identified my 'Early lithionite granite', subsequently renamed the 'Megacrystic Li-mica granite,) was based on optical properties. If it is now believed that the Li-mica has resulted from a metasomatic alteration of biotite, an explanation for the changes giving rise to its different optical properties is required. As to questions, first, if, as I understand it, the fluorite granite is now thought to be due to alteration of the Li-mica granite, where has the iron gone? The Li-mica is distinctly dark but no dark minerals occur in the fluorite granite. And, secondly, is the usage of 'gilbertisation' synonymous with my term 'greisening', meaning the diffuse and pervasive breakdown of K-feldspars to white mica and quartz? Authors' reply: Before answering the questions, we must emphasize that our view of the alteration of lithium-mica granite to fluorite- and gilbertite-bearing material as being a late-magmatic process is based solely on field observation. This kind of alteration is commonly seen in the end stages of pegmatite development. We consider that fluids responsible for the concentration of fluorine probably rendered the lithium-mica unstable and brought about its replacement by stable greenish muscovite (gilbertite), - a change involving rearrangement of silicon and aluminium and loss of magnesium and lithium as well as of iron. As the system cooled, fluids presumably carried unwanted elements (including iron) into surrounding aureole rocks. The term 'gilbertisation' is used only to denote alteration of original lithium-mica to greenish muscovite which can be readily observed in the field. At this stage of the work, we feel that the process was independent of later hydrothermal processes responsible for greisening associated with quartz-tourmaline veining, lode-mineralisation and general kaolinisation of the granite.

Radioactive disequilibrium in uranium-bearing nodules from the New Red Sandstone (Permian- Triassic) of Budleigh Salterton, Devon E.M. DURRANCE R.E. MEADS R.K. BRINDLEY A.G.W. STARK

Introduction The unusual composition of the dark nodules which occur within the red marls and sandstones of Littleham Cove, at the western end of Budleigh Salterton beach, was first recognised by Carter (1931). Subsequently, detailed studies of these nodules have been carried out by Perutz (1939), Wyley (1961), Harrison (1962, 1975) and Durrance and George (1976). Uranium, vanadium, nickel, cobalt, chromium, copper, arsenic and silver have all been recorded in various amounts, forming minerals such as vanadian mica. niccolite, native copper etc. Uranium concentrations varying between 0.006 and 1.3% have been recorded by Harrison (1975) from different nodules, while autoradiography shows that banding and the localised growth of minerals containing uranium within bands leads to considerable variations in the distribution of the uranium even within single nodules (Durrance and George 1976). Unfortunately X-ray

diffraction studies of the character of the uranium- bearing minerals have proved generally unsuccessful, although electron probe analysis led Harrison (1975) to suggest that coffinite is present as a primary mineral. Only the presence of the secondary uranium mineral metatyuyamunite has been confirmed by X-ray diffraction analysis, but this together with the occurrence of α- cristobalite produces an association which Durrance and George (1976) believed indicates the presence of a Primary uranium hydrocarbon of the thucolite type. The origin of the nodules, which are clearly postdepositional, was considered by Harrison (1975) to lie in precipitation of these minerals from thermal solutions associated with the waning phases of the metalliferous mineralisation of south west England. He believed that sites of precipitation occurred principally at permeable-impermeable interfaces. In contrast, Durrance and George (1976) suggested that precipitation

E.M. Durrance, R.E. Meads, R.K. Brindley and A.G.W. Stark 1980. Radioactive disequilibrium in uranium-bearing nodules from the New Red Sandstone (Permian-Triassic) of Budleigh Salterton. Devon. Proc. Ussher Soc., 5, 81-88. Gamma ray spectrometer studies of the naturally occurring radioactive isotopes which are found in uranium-bearing nodules at Budleigh Salterton, Devon, have been carried out using high energy (upto 4MeV) and low energy (up to 0.1MeV) semi-conductor detectors and a 400 channel energy analyser. Energies associated with isotopes from the decay series of 238U. 235U. 232Th and 50V have been identified, with those from the 238U decay series contributing the major part of the total radioactivity. Relative peak intensities for 234Th, 226Ra, 214Pb and 210Pb within the 238U decay series have been compared with the theoretical equilibrium state, and disequilibrium found to be present at the 222Rn position. Peak areas produced by 234Th and 226Ra. after correction, are very similar at about5.3 x 105 channel counts but those produced by 214Pb and 2t0Pb are considerably reduced from this value at 9.8 x 104 and 2.7 x 105 channel counts respectively. These values indicate an 80% disequilibrium at the 214Pb position and a 50% disequilibrium at 210Pb. Only the 210Pb disequilibrium is, however, indicative of the in situ condition of the nodules, as the short half-life of 214Pb renders this part of the decay series very susceptible to short term disequilibrium produced by extraction and preparation of samples. Similar investigations using solid state detection of alpha particle energies provide corroboratory evidence for substantial disequilibrium in the 238U decay series at the 222Rn position. Knowledge of a long term disequilibrium of about 500% allows correction of the gamma determined uranium content of about 0.3% to closely approximate the chemically determined value of about 0.5% for the nodule analysed by Perutz (1939); and shows that the stream water radon anomaly centred around the known occurrence of the nodules probably arises by radon diffusing from the nodules. E.M. Durrance, R.K. Brindley and A.G. W. Stark, Department of Geology, University of Exeter, Devon; R.E. Meads, Department of Physics, University of Exeter, Devon

was controlled by fragments of contemporaneous plant material which were buried within the New Red Sandstone sediments. Details of the effect of plant debris on the diagenetic history of the sediments was given by Durrance and others (1978). A significant radon anomaly is found in stream waters in the area around the mouth of the River Exe, close to the known occurrence of the nodules (Durrance 1978). This may be caused either by the last vestiges of the mineralising source, or by the breakdown of uranium -bearing minerals either in the nodules or dispersed more generally within the sedimentary succession (Henson 1971, 1973). Although possessing a natural radioactivity normally sufficiently intense to allow the production of contact autoradiographs in about 14 days using ordinary photographic paper, details of the source of the radioactivity have proved difficult to obtain. Chemical analysis (Carter 1931)indicated that V205 may be present in quantities up to about 14%. Of the other elements with naturally occurring radioactive isotopes no thorium or rare earths were found, and U308 only at the 0.07% level. Perutz (1939), however, has shown that on powdering the nodules and heating the powder at low pressure, the radioactivity is much reduced, but with time increases at a rate which is characteristic of the formation of 222Rn in the 238U decay series. He concluded that most of the radioactivity of the nodules is due to the presence of 238U and its daughter isotopes, and on this basis calculated that the nodules contain about 0.3% uranium. This he compared favourably with a content of about 0.5% uranium determined chemically. The comparison between chemically and radiometrically determined uranium contents made by Perutz (1939) is generally based on the assumption that the uranium decay series is in equilibrium. This means that throughout the decay series in any given period of time the same number of daughter atoms are disintegrating as are being produced by disintegrations of the parent. In the 238U decay series equilibrium will be achieved in about 106 years, once a closed chemical system has been established. In terms of the known ages of mineralisation in South-west England (Edmonds and others 1975) this is an extremely short period of time, so that unless chemical exchange has occurred between the nodules and their host since formation, the assumption that equilibrium has been attained is not unreasonable. However, although the values of 0.5% and 0.3% for the uranium content of the nodules are close in absolute terms, there is sufficient dissimilarity to suggest that the uranium decay series is not in equilibrium. Furthermore, as the chemically determined value is the greater, the disequilibrium is likely to be caused by the loss of daughter isotopes between 238U and 214Bi, the decay of 214Bi being the major contributor to the total radioactivity of the 238U decay series.

Between 238U and 214Bi disequilibrium is most likely to arise by the loss of the gaseous isotope 222Rn, a feature which is in accord with the explanation of the stream water radon anomaly in terms of radon diffusion from the nodules. Disequilibrium studies by Harrison (1975) using alpha track analysis showed, however, that the 238U decay series in the nodules was only depleted in 238U itself; no apparent loss of 222Rn and its daughter isotopes was recorded. To resolve this problem, which in turn bears upon the problem of the origin of the stream water radon anomaly, a detailed investigation of the 238U decay series in two of the concentrically banded type of the nodules (Perutz 1939) was carried out using gamma ray spectrometry, with back-up by a study of the alpha particle energy spectrum from a third nodule. Gamma Ray Spectrometry Two Ge(Li) solid state semi-conductor detectors were used for this investigation, one designed for low energy spectral analysis (up to 0.1 MeV) and the other for high energy emissions (up to 4 MeV). Although the low energy detector allowed very good resolution of gamma ray energies, and consequently very accurate determination of peak positions and peak areas, beyond 0.1 MeV, the detector sensitivity was much reduced and become non-linear. In contrast, the high energy detector maintained a high sensitivity and a linear response over a large energy range, but produced less well resolved peaks. Even so the peaks produced by the high energy detector were far superior to those obtained from the best NaI(TI) scintillation detectors (Adams and Gasparini 1970). Output from the Ge(Li) detectors was amplified in two stages and fed into a 400 channel energy analyser. Samples from the nodules were placed adjacent to the detector windows and heavily shielded by lead from any external sources of radiation. Count times in the order of 60 to 70 hours were normally employed, with calibration of the energy analyser carried out at the start and end of each count period. The calibration isotopes used were 57Co (121.94 keV and 136.37 keV) and 22Na (0.511 MeV and 1.28 MeV). A record of the results from the energy analyser was obtained both by photographing the visual display (in 4 groups of 100 channels), and by transfer to magnetic tape for computer analysis. Sample preparation was quite straightforward consisting of the production of flat surfaces by cutting the nodules in half, and of parallel - sided plates either lmm, 2mm or 3mm thick. All cutting of radioactive material was conducted in a fume cupboard of suitable flow rate. Peak Identification Although energies of specific value should theoretically form line spectra, in practice a continuum of energies is obtained by Compton Scattering and the limited resolution of the detectors. Initially, the energy spectrum associated with a simple flat cut surface was investigated,

giving the results shown in Table I. This Table also shows a possible identification of each peak in terms of the known energies of the naturally occurring radioactive isotopes in the 238U decay series (excluding 214Bi). In order to determine the precision with which the energies were recorded, a sample of 226Ra (part of the 238U decay series) was studied under the same conditions. The results from this indicate an overall agreement between measured and theoretical energy values within 0.005 MeV. This range of errors has not been exceeded in assigning possible isotopes to each of the peaks listed in Table 1.

Table 1. Identified Peaks in the 238U decay series (excluding those due to 214B).

Isotope Energy (MeV Energy (MeV, measured) Lederer and others 1967)

210Pb 0.047 0.0465 238U 0.05 0.048 234U 0.054 0.0533

234Tb 0.064 0.0636 230Th 0.068 0.0678 234Th 0.073 0.0698 234Th 0.093 0.0933 226Ra 0.187 0.1857 214Pb 0.243 0.2419 214Pb 0.296 0.2952 214Pb 0.354 0.352

Relative Intensities Although energies corresponding to decays in the 235U and 232Th series and due to 50V and 40K were recorded, they were insufficiently well developed to allow a detailed quantitative analysis to be made. The main radioactive isotope 238U and its daughter products were, however, clearly represented by well defined peaks. To test for disequilibrium in any decay series ideally it is necessary to determine the relative proportions of each of the decay products, but this is not possible for 238U using gamma ray spectrometry, as not all the decay products are good gamma emitters. However, the 238U decay series is conveniently divided into two parts at the 226Ra position, as with a half-life of 1622 years this is the last of the long-lived isotopes before many short-1ived daughters appear in the series. Disequilibrium in the 238U decay series can also be associated with 226Ra, radium remaining in a deposit which has been selectively leached of uranium, and 226Ra is the parent of 222Rn which, as described above, being a gas at ordinary temperatures and pressures can be lost from the decay chain. Relative intensity measurements between peaks belonging to isotopes both following and preceding 226Ra in the decay

series are thus needed to determine whether or not equilibrium has been established and, if not, the character of the disequilibrium. After corrections have been applied for the character of the decay, the area of any particular peak is controlled by the amount of the isotope undergoing decay. Therefore comparison of peak areas also allows a quantitative assessment of the disequilibrium to be made. At least four isotopes are thus needed for quantitative analysis; two from the series between 238U and 226Ra and two from beyond 226Ra in the decay chain. Those chosen for this study as good gamma ray emitters were 234Th, 226Ra, 214Pb and 210Pb. Gamma ray energies associated with these isotopes that were used for comparison were 63.6, 185.7, 241.9 and 46.5 keV respectively. Peak areas for the four energies were obtained in terms of channel counts directly from the output of the energy analyser, as anomalies over a variable background. Direct comparison between these values is, however, meaningless as the character of the decays producing the emissions differs from one isotope to another. Thus corrections for the variation in branching ratio (the proportion of the decay represented by a particular gamma ray energy) and Internal Conversion Coefficient (the proportion of the gamma rays which may be emitted) have to be applied, as well as any corrections for variation in self-absorption characteristics and detector sensitivity. Fortunately, details of the branching ratios and Internal Conversion Coefficients for the isotopes in the 238U decay series are well known (Lederer and others 1967), and within the energy ranges of the detectors given above, the detector sensitivity was found to be uniform. Self-absorption, though, can only be assessed by direct measurement. For this purpose sections I mm, 2mm and 3mm thick were prepared and uncorrected intensities obtained for two 214Pb peaks from each. The results are given in Table 2. From the values shown in this Table it is clear that there is good agreement between the peak ratios for all three sample thicknesses. Self-absorption was, therefore, considered to be insignificant when measurements were made on samples up to 3mm thick. Table 2. Self absorption data Uncorrected Peak Intensity Isotope Energy (meV) 1mm 2mm 3mm 214Pb 0.2419 6 10 15 214Pb 0.2952 12 21 19 Ratio 2.0 2.1 1.9

Results The results from measurements obtained using the low energy detector are only meaningful for peaks due to 234Th and 210Pb, as the energies associated with the other two isotopes lie outside the range of this detector. Nevertheless, for a 2mm section the following results give an interesting insight into the nature of the decay series: 234Th gave an uncorrected peak of 10440 channel counts and a corrected peak of 2.9 x 105 channel counts. 210Pb gave an uncorrected peak of 6100 channel counts and a corrected peak of 1.5 x 105 channel counts. Thus the results from the low energy detector suggest that there is considerable disequilibrium present, a member of the early part of the decay series having twice the abundance of a later member. A reasonable conclusion which can be drawn from this is that there is a 50% disequilibrium in the decay series at the 222Rn position, or that for every two radon daughters produced only one remains contained within the system. Unfortunately, the results from the low energy detector do not indicate whether or not there is any disequilibrium between 238U and 226Ra, although it is unlikely that 226Ra would be present in smaller quantities than 234Th. In contrast to the limited range of the low energy detector, the high energy detector allowed results from all four isotopes to be obtained. Uncorrected and corrected peak areas for a 3mm section are given in Table 3. These values show that for peaks produced by the decay of isotopes preceding 222Rn in the decay series the ratio of corrected peak areas is 1.1, a figure which is sufficiently close to unity to indicate that the decay series is, within experimental error, in equilibrium between 234Th and 226Ra. It is not possible, however, to determine if there is any disequilibrium caused by the selective chemical Joss of 238U. Table 3. 238U equilibrium data for a 3mm section Isotope Energy (MeV) Uncorrected Corrected Peak Area Peak Area (Channel (Channel counts) counts)

234Th 0.0636 18880 5.5 x 105 226Ra 0.1857 16480 5.0 x 105 214Pb 0.2419 7840 9.8 x 104 210Pb 0.0465 10970 2.7 x 105

If an average of the corrected peak areas for 234Th and 226Ra is taken and ratioed against the correct peak areas for 214Pb and 210Pb, values of 5.3 and 1.9 respectively are obtained. These again indicate that considerable disequilibrium is present between 226Ra and 214Pb, and are best explained by loss of 222Rn from the system. The difference between the 80% disequilibrium at 214Pb and the 50% disequilibrium at 210Pb has probably arisen because radon was artificially degassed from the sample during its preparation, and only isotopes with very short half-lives arc present between 222Rn and 214Pb in the decay chain. The 80% disequilibrium must therefore be considered to be artificially high. In contrast, 210Pb has a moderate half-life of about 19 years, so that its decay would have been unaffected by the loss of radon during sample preparation, and the disequilibrium recorded therefore corresponds to the in situ disequilibrium of the nodules. Thus both the 2mm section recorded by the low energy detector and the 3mm section recorded by the high energy detector indicate that the nodules have an in situ disequilibrium of about 50%, caused by the loss of 222Rn. Errors in the values of branching ratio and Internal Conversion Coefficient used for the decays; and in measuring peak areas, result in an accuracy for the value of the disequilibrium estimated at 10%. Alpha Particle Spectrometry Although both low energy and high energy gamma ray detectors gave the same result for samples of different thickness, an additional check on the character of the disequilibrium in the nodules was made by recording the alpha particle energy spectrum from the 238U decay series. Basically the same type of equipment as used for the gamma ray spectrometry was employed, that is solid state detection of alpha energies, amplification and analysis by an energy analyser. However, because alpha particles have a very limited range even in air, both the sample and the detector were placed in a vacuum chamber. The problem of energy toss suffered by alpha particles is even then not overcome, because of serf-absorption caused by the sample, itself. Thus great care is needed in the preparation of the sample. Initially the samples were prepared as sections about 25µ m thick mounted on glass slides, but unfortunately these resulted in the production of a continuum of energies in which individual peaks were unrecognisable. As a result of this failure the sample material was finely powdered (less than 5µ m), spread out on a glass slide and lifted off with Sellotape immediately prior to placing in the detection chamber. Although these powders produced spectra with discrete identifiable peaks a continuum of energies caused by self-absorption still remained, and the count rate was very low. Count times of up to 4 weeks were employed for some measurements. Therefore, before attempting to analyse an alpha particle spectrum from the nodule material the

problem of allowing for serf-absorption effects had to be overcome. This was achieved by recording the spectrum from a sample of uraninite in which the 238U decay series was reasonably likely to be in equilibrium. The result after a count time of 40 hours is shown in Figure 1. Alpha particle energies for the main alpha emitters in the 238U decay series are given in Table 4. These indicate that peaks in the spectrum should occur at approximately 4.2, 4.7, 5.4, 6,0 and 7.7 MeV as it is very unlikely that the detector could distinguish between peaks which have a separation of about 0.1 MeV caused by the three separate isotope decays of 234U, 230Th and 226Ra or the two separate isotope decays of 210Pb and 222Rn. The ratios of areas of peaks produced in these positions should therefore be in the order of the number of constituent isotope decays, assuming that the decay series is in Table 4. Alpha particle emitters and energies in the 23H[I decay series Isotope Energy (MeV, Expected Peak Leader and others Energy (MeV) 1967) 238U 4.19 ) 4.2 4.15 ) 234U 4.77 ) 4.72 ) ) 230Th 4.69 ) 4.7 4.62 ) ) 226Ra 4.78 ) 210Pb 5.31 ) 5.4 222Rn 5.49 ) 218Po 6.00 6.0 214Po 7.69 7.7 equilibrium. Examination of the spectrum shown in Figure 1 shows that this pattern is indeed developed, but that the peaks are skewed towards the low energy end of the spectrum. This skewness is particularly well illustrated by the peak due to 214Pb, as it lies outside the zone of interference caused by other peaks. If the shape of this peak is taken as the standard response to all other alpha emissions, a theoretical spectrum for the 238U decay series in equilibrium can be constructed and compared with the measured spectrum. This is also shown in Figure I and clearly indicates how closely the uraninite sample conforms to the theoretical equilibrium condition. This is important because not only is a spectrum produced Which can act as a standard against which spectra from the nodule material can be compared, but it also indicates that provided the samples are prepared immediately before counting commences the procedure of powdering need not disturb equilibrium conditions if the material is reasonably compact.

Because of the comparatively low concentration of uranium in the nodules, much longer count times were needed to obtain spectra of magnitude comparable to that from the uraninite. A typical spectrum from the nodule material, formed after a count time of 5 days, is shown in Figure 2. This spectrum contains a number of features which differ from those displayed by the uraninite spectrum. For instance, at the low energy end of the spectrum the peak due to 238U is reasonably clearly resolved and separation of the peak associated with 230Th from the coalesced peaks of 234U and 226Ra is apparent. Also, the peak due to 222Rn is here the dominant feature of the spectrum. In contrast, at the high energy end 0fthe spectrum the peaks resulting from 218Pb and 214Pb are very much reduced in size compared with the peaks from the low energy range, even when the radon peak is excluded. Finally, the peak due to 210Pb appears to be hidden by the large 222Rn peak. The features of this spectrum may, of course, be explained in terms of a disequilibrium in the 238U decay series. Clearly, the exceptional size of the 222Rn peak must have resulted from a large amount of radon diffusing from the sample and entering the vacuum chamber during the counting period. This means that alpha particles from the radon which left all surfaces of the powdered sample were counted, whereas only those alpha particles which emerged from surfaces aligned with the detector window were counted for the other isotopes. The largest anomalous feature of this spectrum thus probably arose through disequilibrium induced by the procedure of sample preparation of very open-textured material. Nevertheless, an examination of the remaining sections of the spectrum does allow an assessment of the pre-sample preparation or in situ disequilibrium to be made. Compared with the spectrum from the uraninite, the nodule material produced a spectrum in which the peaks due to 214Pb and 218Pb are much reduced in size in relation to those resulting from 238U, 230Th, 234U and 226Ra. Unfortunately, the ratio of peak areas from isotopes before and after turn in the decay series does not directly give the in situ disequilibrium as, with the exception of 210Pb, all alpha particle decays in the series beyond 222Rn have very short half-lives, and their associated peaks may in part have resulted from the decay of the radon released into the vacuum chamber. Only 210Pb which has a half-life of 138 days would be suitable, but this peak is hidden. However, the ratio of the peak areas does indicate the minimum in situ disequilibrium if it is assumed that this is the only way in which the peaks result. Therefore, as the ratio of the peak areas is about 2:1, a minimum of 50% disequilibrium for the nodules is suggested. Furthermore, comparison of the peak areas from isotopes between 238U and 226Ra shows that all are very similar. Therefore the decay series is likely to be in equilibrium until the 222Rn position is reached. Clearly, the results of this alpha particle spectrometry study generally agree with those obtained

Figure 1. Solid line: alpha particle spectrum of uraninite (238U decay series). Count period 40 hours. Broken line: alpha particle spectrum for a 238U decay series in equilibrium.

using gamma ray spectrometry, and suggest that in the nodules the 238U decay series is in equilibrium until 222Rn is reached, when approximately a 50% disequilibrium occurs. Conclusions The results of this study of radioactive disequilibrium are interesting in two ways. Firstly, the source of the radon recorded by Durrance (1978) is now seen to probably lie in the loss of gas from the nodules themselves, rather than from the breakdown of uranium-bearing minerals more generally dispersed within the sediments of the New Red Sandstone, and secondly, the recognition that there is an in situ disequilibrium of about 50% in the nodules allows a correction to be made to the uranium content of the nodules as determined by their radioactivity. In the gamma ray spectrum of the 238U decay series the major emissions are due to the decay of 214Bi. Any determination of the uranium content using the total radioactivity is thus largely based on the amount of 214Bi present, end the assumption that the decay series is in equilibrium. However, with a 50% disequilibrium due to loss of 222Rn this method underestimates by something approaching 50% the actual uranium content. Therefore the difference between the values of 0.3% uranium determined from radioactivity measurements and 0.5% uranium determined by chemical analysis, given by Perutz (1939), could be a reflection of this degree of disequilibrium. The chemically determined uranium content is thus likely to be correct, and because individual nodules different from that of Perutz (1939) were investigated in this study, the agreement obtained may indicate that this degree of disequilibrium is a general characteristic of the nodules. References Adams, J. and Gasparini, P. 1970 Gamma ray spectrometry of

rocks. Amsterdam. Elsevier. Carter, G.E.L. 1931. An occurrence of vanadiferous nodules in

the Permian beds of South Devon. Mineralog. Mag., 22, 609 -613.

Durrance, E.M. 1978. Radon in the stream waters of East Devon. Proc. Ussher Soc., 4, 220-228.

Durrance, E.M. and George, M.C. 1976. Metatyuyamunite from the uraniferous - vanadiferous nodules in the Permian marls and sandstones of Budleigh Salterton, Devon. Proc. Ussher Soc., 3, 435-440.

Durrance. E.M., Meads, R.E., Ballard. R.R.B. and Walsh, J.N. 1978. Oxidation state of iron in the Littleham Mudstone Formation of the New Red Sandstone Series (Permian-Triassic) of south east Devon. England. Geol. Soc. America. Bull., 89, 1231-1240.

Edmonds, E.A., McKeown, M.C. and Williams. M. 1975. British regional geology: South-West England. London, Her Majesty's Stationary Office.

Harrison, R.K. 1962. The petrography and mineralogy of some sedimentary nodular structures. M. Sc. Thesis (unpublished). University of Readlng.

Harrison, R.K. 1975. Concretionary concentrations of the rarer elements in Permo-Triassic red beds of south-west England. Geol. Survey Great Britain, Bull., 52, 1-26.

Henson, M.R. 1971. The Permo-Triassic rocks of South Devon. Ph.D. Thesis (unpublished), University of Exeter.

Henson, M.R. 1973. Clay minerals from the Lower New Red Sandstone of South Devon. Proc. Geol. Assoc., 84, 1429-445.

Lederer, C.M., Hollander, J. and Perlman, I. 1967. Table of Isotopes. Wiley, New York.

Perutz, M. 1939. Radioactive nodules from Devonshire, England. Mineral. Petrog. Mitteil., 51, 141-161.

Wyley, J.F. 1961. Metatyuyamunite from the Permian, Budleigh Salterton, Devon. Not. Miner. Soc., No. 112, 19-1-1961.

Note on a fossiliferous Meadfoot Group locality at Punch's Cross, Polruan, Cornwall M. EVANS

Department of Geology, National Museum of Wales, Cardiff CFI 3NP Peach (1865, p.24) referred to a fossiliferous locality at Punch's Cross near Polruan, Cornwall. The Geological Survey memoir (Ussher and others 1909) included a long faunal list from Punch's Cross quarry but also noted that "the name is not found on the maps". House and Selwood (1966) referred to the "now lost locality of Punch's Cross" but did not explain why the locality is no longer accessible.

The spelling varies in the literature, Punch's, Punch and Punchey's Cross having been quoted. The list of brachiopods given by Ussher and others (1909, p.19) is typical of the Meadfoot Group (Harwood 1976) in the area around Fowey and Looe.

On the 1:10560 Ordnance Survey map (Sheet SX 15 SW), the name "Punch Cross" is recorded on the east side of the Fowey estuary (SX 12255080), south of Punch rocks. In fact, the map refers to a rocky peninsula bearing a large, ornamental cross which marks the limit of the parish boundary.

Punch's Cross quarry was situated in Polruan (SX12305090) but, in 1922, a house was built within the old quarry. At the invitation of the owner, Mr J. Hill. the present writer was able to examine the foundations of the house ("Headlands", Battery Lane, Polruan) and can confirm that the rocks are typical of the Meadfoot Group of the Fowey area, although no fossils were found.

Although the Punch's Cross locality is now lost to palaeontologists, it is possible to collect material from virtually the same horizon on the shore at Polruan (SX12255085) and across the estuary at Fowey (SX12145134, 12305140). Collections made by the writer yielded the following brachiopod fauna- Platyorthis circularis (Sowerby), Proschizophoria personata (Zeiler), Schizophoria provulvaria (Maurer), Oligoptycner-hynchus daleidensis (Roemer), Acrospirifer primaevus (Steininger), Hyterolites hystericus (Schlotheim) and Athyris avirostris (Krantz).

The fossil preservation is poor, in the form of internal and external moulds in bands of yellowish-brown, friable sandstone. The brachiopod fauna is considered, by the present writer, to indicate a Late Siegenian age and is consistent with other collections from the Meadfoot Group of north Cornwall.

Acknowledgements. The writer would like to express particular thanks to Mr Hill for his assistance with the history of Punch's Cross quarry and his permission to examine the foundations. Thanks are also due to Mr G. Bisson and the staff at IGS Exeter office for access to the field maps and notes of Ussher. This work was carried out during a three year NERC grant under the supervision of Dr V.G. Walmsley. Harwood, G.M. 1976. The Staddon Grits - or Meadfoot Beds?

Proc: Ussher Soc., 3, 333-338. House, M.R. and Selwood, E.B. 1966. Palaeozoic

palaeontology in Devon and Cornwall. In Present views on some aspects of the Geology of Cornwall and Devon. R. Geol. Soc. Cornwall, Commem. Vol. for 1964, 45-86.

Peach, C.W. 1865. On the Fossil Geology of Lantivet and Lantic Bays, near Fowey. Trans. R. Geol. Soc. Cornwall, 7, 17-27, 2 pls.

Ussher, W.A.E., Barrow, G. and MacAlister, D.A, 1909. The geo logy of the country around Bodmin and St Austell (Sheet · 347). Mem. Geol. Surv., 1-201, 3 pls. H.M.S.O. London.

Read at the Annual Conference of the Ussher Society, January 1980

Palynological studies of Triassic rocks in Central Somerset (Abstract) G. WARRINGTON Institute of Geological Sciences, Ring Road Halton, Leeds LS15 8TQ

Late Triassic (Carnian) miospores occur in samples from saliferous beds in the Mercia Mudstone Group (formerly 'Keuper Marl') proved, in 1910, in the Puriton Borehole, near Bridgwater. A correlative saliferous unit proved in that group in the Institute of Geological Sciences' Burton Row Borehole, Brent Knoll, has not yielded palynomorphs but its age can be interpolated from the results from the nearby Puriton section. The lowest palynomorph assemblage from the Mercia Mudstone Group of the Burton Row Borehole comprised late Triassic (Rhaetian) miospores from 74m below the Penarth Group (formerly 'Rhaetic'). Miospore associations increase in diversity towards the top of the Mercia Mudstone Group and are joined by organic- walled microplankton 2 metres below that horizon. Palynomorph assemblages from the Penarth Group include numerous organic-walled microplankton, principally dinoflagel!ate cysts, and are augmented by scolecodonts and test-linings of foraminifera. The diversity of these assemblages declines abruptly at the top of the Penarth Group and those from the Lias are comparatively restricted in composition. The palynomorph succession of the upper Mercia Mudstone Group to basal Lias sequence in the Burton Row Borehole compares closely with that from the same lithostratigraphical units exposed On the west Somerset coast near Watchet. This contribution is published by permission of the Director, Institute of Geological Sciences.

British Triassic palaeontology: supplement 4

G. WARRINGTON Institute of Geological Sciences, Ring Road Halton, Leeds LSI5 8 TQ Since the submission of the writer's previous supplement (Proc. Ussher Soc., 4, 297-8; 1979) to his paper on British Triassic palaeontology the following works dealing with or including aspects of that subject have appeared: Antia D.D.J. 1979. Bone-beds: A review of their classification,

occurrence, genesis, diagenesis, geochemistry, palaeocology, weathering, and microbiotas. Mercian Geol., 7, (2), 93-174.

Bate, R.H. 1978, The Trias, pp. 175-188 in Bate, R.H, and Robinson, E. (eds), A stratigraphical index of British Ostracoda. Geological Journal Special Issue No. 8, Seel House Press, Liverpool, 538 pp.

Duffin, C.J. 1978. Tropifer laevis Gould (Coleiidae: Crustacea) and a new crustacean from the Rhaetian Bone Bed of Aust Cliff, Avon. Zool. J. Linn. Soc., 64, 177-185.

Duffin, C.J. 1979. Coprolites: A brief review with reference to specimens from the Rhaetic Bone-Beds of England and South Wales. Mercian Geol., 7, (3), 191-204.

Morbey, S.J. 1978. Late Triassic and early Jurassic subsurface palyn0stratigraphy in northwestern Europe. Palinologia, num. extraord. 1,355-365.

Owens, B. and Marshall, J. (Compilers). 1978. Micropalaeontological bi0stratigraphy of samples from around the coasts, of Scotland. Rep. Inst. Geol. Sci., No. 78/20, ii + 35pp.

Pollard, J.E. and Steel, R.J. 1978. Intertidal sediments in the Auchenhew Beds (Triassic) of Arran. Scott. J. Geol., 14, (4), 317-328.

Poole, E.G. 1979. The Triassic-Jurassic boundary in Great Britain. Geol. Mag., 116, (4), 303-311.

Sykes, J.H. 1979. Lepidotes sp: Rhaetian fish teeth from Barnstone, Nottinghamshire. Mercian Geol., 7, (2), 85-91.

This contribution is published by permission of the Director, Institute of Geological Sciences.

Heavy minerals in the Upper Albian/Lower Cenomanian P.J. LOVELAND Soil Survey of England and Wales, Rothamsted Experimental Station, Harpenden, Herts. Hancock (1969) showed that two mineralogical provinces exist in the Cenomanian of England viz: a south-west province dominated by tourmaline; and a north-east province with much less tourmaline and more zircon and garnet. His conclusions were based on samples taken as far east as West Dorset, but he postulated that the north-east province should extend further east, across the rest of England. Data on the heavy minerals of the Upper Albian (dispar zone)/Lower Cenomanian (mantelli zone) obtained during an investigation of soils developed in glauconitic parent materials (Loveland 1978) support this postulate. The heavy mineral data are summarised in Table 1 with some of Hancock's (1969) data for comparison. The ratios of some of the more significant members of the mineral assemblage are also shown. The lithological units were dated by reference to Rawson and others (1978).

It can be seen clearly that the decrease in tourmaline which is a principal feature of Hancock's north-eastern

province in Dorset, does indeed continue to the east. There is, however, relatively little difference between the tourmaline: zircon: garnet ratios of the Upper Albian and Lower Cenomanian of the north-east province, although the Upper Albian tends to contain more garnet. Some of this has clearly been incorporated into the Lower Cenomanian, a point noticed by Hancock. The absence of garnet from the Albian and Cenomanian of the southwest in contrast to the north-east indicates that the source rocks are indeed different for these two provinces. Furthermore, the barrier between these two regions of sedimentation effectively existed in Albian times. Hancock, J.M. (1969). Transgression of the Cretaceous Sea in

South-West England. Proc. Ussher Soc., 2, 61-83. Rawson, P.F., Curry, D., Dilley, F.C., Hancock, J.M., Kennedy,

W.J., Neale, J.W., Wood, C.J. and Worssam, B.C. 1978. A correlation of Cretaceous rocks in the British Isles. Geol. Soc. Lond. Special Report No. 9, 70pp.

Loveland, P.J. 1978. An investigation into the nature and genesis of some glauconitic soils in central southern England. Unpub. Ph.D. Thesis, University of London, 354pp.

Table 1. Percentages of Non-opaque Heavy Minerals and Ratios of Tourmaline (T): Zircon (Z): Garnet (G)

+= Hancock (1969) ; a = one grain reported in traverse

Read at the Annual Conference of the Ussher Society, January 1980

An early Flandrian sea-level in the Severn Estuary (Abstract) D.D. GILBERTSON 1

A.B. HAWKINS2

1 Department of Archaeology and Prehistory, University of Sheffield 2 Department of Geology, University of Bristol

Fine examples of submerged forests may be observed one and a half kilometres seaward of HWST mark off the ICI industrial complex, Severnside, at approximately -5.5 m O.D. These deposits, only exposed at the lowest tides of the year, have been investigated in the field, sampled, and their age and environmental significance identified by radio-carbon dating supported by pollen-analytical and molluscan studies. The Flandrian sequence rests directly on Keuper Marl bedrock, and consists of four repeated sequences comprising trees overlain by fresh and/or brackish water peats, overlain by inter-tidal deposits. Oak (Quercus sp.,) dominated woodland with significant proportions of Ulmus, Tilia, Betula and Pinus, was engulfed by freshwater carr and swamp, as a result of land and freshwater stream drainage impedence caused by a rising sea level. Inter-tidal salt marsh or tidal flat deposits were then deposited over the peats, before the area was re-colonised by mixed oak woodland, as a result of a minor marine transgression and/or the formation of a coastal barrier which temporarily excluded the sea. The centre of a tree rooted in the second oldest peat has been dated to 7030 ± 115 yrs b.p. (I 4903). This stump immediately pre-dated the onset of drainage impedence caused by the rising sea level. Possibilities of contamination cannot be excluded in this environment, The maximum tidal range in the present estuary at this location is about 14m. These data indicate that shortly after 7000 yrs b.p., mean sea level in the upper Severn Estuary was at approximately -12 to -13 m O.D.

Read at the Annual Conference of the Ussher Society, January 1980 Geomorphology of Camel Valley and Estuary (Abstract)

B.B. CLARKE

Department of Extra-Mural Studies, University of Exeter, 5 Walsingham Place, Truro TRI 2RP The Camel and tributaries drain three units: 1. The High Moor, mostly granite, 170-300m, with summit-tors rising to 400m. Roughtor, the largest, has keeled tor-stones with a higher apron of scree and a lower of fallen tor-stones. Devils Jump is a valley-side tor. The surface has low relief, meandering streams and widely developed blanket-bog. Above the Camel at Trecarne is a bench at 200m, possibly a transgressive late Tertiary marine feature, but Simpson (1964) considers features at 210m in Devon may be exhumed Upper Cretaceous 2. A Coastal Plateau of folded slate and greenstone, 55-170m, the 130m marine platform, falling to a lower younger one near the coast. The, spurless valleys deeply dissect the old platform, and have steep forested sides, while the interfluves are wide, gently sloping and cultiva-ted. Rejuvenation is tentatively correlated with glacial low sea-levels. Below Penhargard the Camel flows through 3km of gorge 60m deep. Weller (196!) suggests the contrasting wide valley of Halgavour Brook may once have carried the whole drainage south. 3. The Estuary is a ria, 15km long, a Flandrian rise in sea-level having flooded the lower valley. The sides, however, are benched, testifying to an earlier Pleistocene history. Old beach sands and gravel survive on the former shore platforms at Daymer Bay, Trebetherick Point, Pentire Haven and Pentire where the beach deposit is lm thick and contains boulders 0.5m across. Two deposits of head testify to periglacial frost shattering and are soliflucted down to the platform, sealing the older beach. Dune sand, which covers the head, is in places cemented to eolianite and occasionally rests directly on the raised platform. Perched boulder gravel; high above the platform may indicate intrusion by ice. At Trebetherick a fragment of gravel suggests recessional moraine outwash, and at Tregunna faceted boulders in clay suggest marginal moraine, while at Trewornan a lake flat could signify glacier impounding. Arkell (1943) and Kidson and Tooley (1977) however suggest alternative views. At St Saviours Point, resting on the raised platform is a ferricrete beach of rounded gravel and coarse head thoroughly mixed suggesting a second younger beach formation near the end of Older Head times. Compare the views of Mitchell and Orme on the Scillies (1967) and Coutard and others (1979) on Normandy.

Arkell, W.J. 1943. The Pleistocene rocks of Trebetherick Point,

North Cornwall, their interpretation and correlation. Proc. Geol. Assoc. 54, 141-170.

Coutard, J.P., Lautridou, J.P., Lefebvre, D. and Clet, M. 1979. Les Bas-Niveaux Marins Eemien et Pre-Eemien de Grandcamp-les-Bains. Bull. Soc. Linn. Normandie, 107, 11-20.

Kidson, C. and Tooley, M.J. 1977. Quaternary History of the Irish Sea, Wiley, Chichester.

Mitchell, G.F. and Orme, A.R. 1967. The Pleistocene deposits of the Scilly Isles Q.J. Geol. Soc. Lond., 123, 59-92. Simpson, S. 1964. The supposed 690ft marine platform in Devon. Proc. Ussher Soc., 1, 89-91.

Weller, M,R. 196 I. Palaeogeography of the 430ft shoreline stage in East Cornwall. Abstracts. Proc. Conf. Geol. and Geomorphol., S.W. England, 23-24.

Read at the Annual Conference of the Ussher Society, January 1980

A preliminary report on the use of terrestrial photogrammetry in the study of rock slopes in Cornwall K. ATKINSON1 P.C. STETHRIDGE2

1Camborne School of Mines, Pool, Redruth, Cornwall 2Cornwall County Council Highways Laboratory,' Radnor Road, Scorrier, Redruth, Cornwall

Introduction Terrestrial photogrammetry is being used to aid the study of rock slope failures in different rock types at three sites in Cornwall. Because of the inaccessibility of these sites (at Mullion, Mevagissey and Samphire Island, south of Portreath) it was either impossible or difficult to use normal survey and discontinuity measurement techniques, so that it was decided to use terrestrial photogrammetry as a remote measuring technique to provide data for discontinuity analysis, measurement of rates of erosion and the production of scaled sections, elevations and plans.

Theory of terrestrial photogrammetry A pair of photographs, with the plates at a known orientation to the base line, is taken at either end of a fixed base line. A three-dimensional image of the subject, in this case a cliff face, can then be reconstructed using either a stereo comparator or a stereo plotter. Figure 1 shows the field set-up of a pair of parallel, vertical plates. The overlap area S depends on the ratio of the distance between the base line and the face Y, and the length of the base B. The ratio of B to Y should be between 1 to 5 and 1 to 20.

The measurement of the co-ordinates of any point on the face can be made using either a stereo comparator or a stereo plotter. In the case of the stereo comparator once the parallax readings are taken from the photographs the co-ordinates are calculated mathematically taking into account the relative orientations of the plates. In the case of a stereo plotter these orientations are carried out mechanically and the co-ordinates read directly from the machine. Since a comparator has been used in this project it is this system that will be described in more detail. The fudicial marks together with the photo number and focal length of the lens are marked on each plate. The lines joining the. fudicial marks form the x and y axes of the photograph and their intersection point, also marked on the photograph, is on the principal axis of the camera, line L O P Fig. 2. With the plates set up in the stereo comparator and the point to be measured chosen, four measurements are made, these being the x and y co-ordinates of the point

being viewed on the left-hand plate and the x and y parallaxes between the left-hand and right-hand plates. The orientation of the plates in Fig. 2 shows the simplest possible configuration. It can be seen that by some elementary trigonometry using the parallax readings, the co-ordinates of point A relative to the Global axes whose origin is at the left-hand end of the base line - i.e. L - can be calculated. However, in practice, in order to get the subject to be photographed into view it is often necessary to set the plates at angles other than vertical and parallel with the x axes, i.e. tilted and parallel averted. Also, there is normally a difference in level between the principal points of the photographs. With this more general set of conditions, the majority of which occur in practice, the only practical way to tackle the solution of the geometrical calculations involved requires the use of a computer. Although a programmable pocket calculator can be used for the calculations it cannot overcome the problems of data handling and storage. Hence, in order to produce co-ordinates or dip and dip directions of discontinuities using terrestrial photogrammetry, the following procedure is used:-

SELECT AREA TO BE STUDIED

TAKE PHOTOGRAPHS

SET UP PHOTOGRAPHS IN STEREO COMPARATOR

TRANSFER PARALLAX DATA TO COMPUTER CALCULATE COORDINATES

and/or DIP AND DOP DIRECTION

Equipment Field equipment A Wild P31 Universal Terrestrial camera is used. Two sets of legs are set up - one at each end of the base line. The camera is set up at one end of the base line with a sighting target at the other and the first set of photographs is taken. The positions are then reversed. A 20 second Wild theodolite is used to survey the face targets. The total weight of the equipment is approximately 90lbs. Normally six targets are set up and surveyed relative to the base line. The co-ordinates of the targets calculated from both the theodolite survey and the stereo pairs are compared in order to check the accuracy and correctness of the field measurements and the laboratory analysis of the photogrammetry work. Laboratory equipment A Carl Zeiss Jena 1818 Stereo Comparator was used fitted with a Whitwen Data Systems Digitiser which gives an optical electronic readout of the parallax readings together with the point number. This information can be transferred by hand to a data form for punching. If there is a large quantity of data, as in the case of this project, it can be transferred directly onto punched tape. This is carried out by transferring the data via the digitiser and an interdata 7/16 mini computer to a teletype terminal with a tape punch. The data are controlled and put into the correct format by the computer control program. The sending of the data from the digitiser is controlled by a footswitch. Hence, once the stereo comparator is adjusted by the operator on to the required point, the footswitch is depressed and the information punched onto the tape which is read into a computer file where it is edited to remove any errors and then run with the main analysis program.

Accuracy of the co-ordinate measurements The accuracy with which co-ordinates on the face can be measured depends on a number of factors which include:- Accuracy of field measurement (a) height of camera axis above base, (b) survey of base length and the difference in level between the two stations, (c) levelling of camera mount. Accuracy of camera and stereo comparator The theoretical accuracy of the measurement of the object distance y depends on the base to distance ratio b and the parallax error dpx. y i.e. theoretical error dy = + y2 dpx b.f. where f is the principal distance. It can be se en that dy increases with the square of the object distance y whereas the errors in the x and z plane only depend upon the scale. The theoretical size of the parallax error dpx depends on the accuracy of the stereo comparator which is 1 micron, the dimensional stability of the photographic prints, and the accuracy of the camera. The value of dpx is assumed to be 7 microns and f = 100mm. Ability of Operator It is in this area that significant errors can arise, both in the setting up and measuring from the stereo comparator. The operator accuracy is dependent upon practice, concentration and good eyesight. Discontinuity analysis When a discontinuity analysis is carried out the dip and dip direction of the planes of the discontinuities on the face are measured along a series of horizontal and vertical scanlines. Three points are measured on each plane. The analysis program calculates the co-ordinates of the three points, fits a plane through them, and gives the dip and dip direction. The results are plotted on an equal area stereogram and contoured. Structural analysis of the joints represented by the high pole concentrations can then be carried out. From the work that has been carried out so far it has been found that this analysis method gives a wider scatter of results than a control field survey. This scatter is thought to be due to the following reasons:- (a) The difficulty of taking parallax measurements on planes that are at or near 90o to the plates i.e. the strike of the plane is along the y axis. (b) False readings being taken because of the shadows cast by some steeply dipping planes.

(c) One or more of the three points chosen on the plane is not representative of the overall dip and dip direction. (d) The accuracy is dependent on the spacing of the three points. The closer they are together the greater is the effect of the limits of accuracy of the system. The first two problems can be overcome by taking more than one set of photographs and by taking the photographs in bright overcast conditions. With regard to the third and fourth problems the authors are investigating the effect of replacing the present method of calculating dip and dip direction with a system of measuring more than three points and calculating a trend surface using regression analysis. This method has the advantage of giving a measure of the lack of fit of the plane through the points, also the number of points measured on any particular plane can be varied to suit the circumstances. Examples (a) Mullion The site is located 0.8km south of Polurrian Cove on the Lizard peninsula. The bedrock consists of Hornblende Schists. At 'the site there is a major fault zone running inland from the shore line. Weathering has taken place to a considerable depth along the fault zone and erosion has resulted in a steep sided narrow valley. In addition the rock is intersected by a number of joint sets. A terrestrial photogrammetry survey has been made of the eastern face together with a field discontinuity survey of the accessible parts of the face. A discontinuity survey has been carried out along a series of horizontal and vertical scan lines. A scaled plan, elevation, and sections of this face are in the course of preparation. (b) Samphire Island The site is located 1.6km south of Portreath where a section of cliff is being rapidly eroded by the sea. The bedrock of the area consists of the Devonian Portscatho Series of sandstones and slates. Terrestrial photogrammetry is being used to study the rate of erosion of the cliff. The first set of photographs was taken in January 1978 and it is planned to take a second set in the spring of 1980. In addition it is hoped that further photographs will be taken at yearly intervals as part of a continuing project. A scaled plan, elevation, and sections are being prepared from the photographs and hence the rate of erosion can be calculated by comparison with later sets of photographs. (c) Mevagissey The site lies on the cliffs 0.8km north of Mevagissey. The bedrock consists of the Lower Devonian Grampound Grit which at this site consists of black slates with quartz veins. Rapid erosion producing a series of wedge failures has taken place. As part of the study of this site it is

planned to carry out a back analysis of the wedge failures, A field survey of the lower part of some of the Wedges has been carried out but. with some difficulty. Terrestrial photogrammetry is being used to determine the geometry of the discontinuities producing the wedge failures. Conclusions In this study terrestrial photogrammetry is being used to provide data for: (i) The measurement of rates of erosion (ii) The production of scaled sections, elevations, and plans (iii) Discontinuity analysis In the ease of (i) and (ii), the method promises to give satisfactory data for analysis. With regard to discontinuity analysis, using photogrammetry data from the site at Mullion, it has been found that the pole plot of the results from a series of horizontal scanlines on the photographs gives a much wider scatter of results than a field survey of the accessible parts of the same face. The authors are investigating the possible reasons for this difference including the effect of calculating the dip and dip direction of the plane by measuring three or more points on the surface and fitting a trend surface using regression analysis. At present three points are measured and a plane is fitted through them. Hence it is not yet known whether terrestrial photogrammetry can be used with confidence in this study to provide data for discontinuity analysis. It has been found that the two principal advantages of terrestrial photogrammetry over conventional surveying techniques are: (a) Measurements can be made on otherwise inaccessible faces, (b) The majority of the time involved is spent in the analysis of the photographs, fewer site visits are required and the method is less dependent on weather conditions. Written discussion Dr C.M.L. Bowler: Could the marked differences between the field observations and the photographic observations of discontinuity orientation be at least partly explained by differences of expectation and perception? For instance, in the field could we expect a selection according to the likely importance (as suggested by fracture persistence or by fracture width) of the several groups of discontinuities and in the photographic interpretation surely shadows could be significant? Does the amount and angle of sunlight on the face make a difference? Dr P.A. Floyd: The different stereogram distribution patterns for the measured relative to the photographed surface data may well be due to the unconscious selection of specific surfaces by the field operator.

Authors' reply to Dr Bowler and Dr Floyd: The selection of discontinuities is always to a certain extent subjective and that selection, when the face is accessible, is probably more representative in the field than .using photogrammetry. The difficulties of using photogrammetry increase when the face contains a large number of closely spaced discontinuities producing small planes on which the measurements have to be made. This was the case with the face at Mullion on which the comparison between the field and photogrammetry measurements was made. The amount and angle of the sunlight does have an effect especially if long shadows are cast. The ideal conditions are bright overcast weather or in sunlight with the sun's rays normal or near normal to the face. Dr M.R. Harvey: (a) Considering that the study is spread over a number of years in order to monitor and to witness rock slope failure in areas where you have studied and photographed would it not be an idea to investigate your techniques in a working quarry such as the china clay pits in the St Austell area? Erosion, albeit artificial, takes place at a much faster rate and could possibly be "designed to order" such that your predictions might be witnessed in a much shorter time span. (b) Since many rock slope failures are rotational very often following an arcuate crack pattern developed in the body of the rock, often only manifesting themselves at the top of the cliff, I should think it would be difficult to observe and especially to measure the directions of any of these cracks by photogrammetry. Authors' reply: (a) The possibility of using a clay pit face was considered but the cliff face at Samphire Island is being eroded at a high enough rate for significant changes to have taken place since the first set of photographs was taken in January 1978. (b) Failures in moderately and highly weathered rocks, often found in the higher sections of cliff faces, are sometimes rotational in nature. These types of failures cannot be predicted by discontinuity analysis methods since the failure surface is not dependent on the orientation of the discontinuities. However photogrammetry can be used to measure the dimensions of the slip surface after the failure has taken place if a 'back analysis' is required.

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