Deep Alteration of Carboniferous Strata in the Midleton, Co. Cork District as Detected by Gravity...

Deep Alteration of Carboniferous Strata in the Midleton, Co. Cork District as Detected byGravity SurveyingAuthor(s): Thomas MurphySource: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, andChemical Science, Vol. 64 (1964 - 1966), pp. 323-334Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20494893 .

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[ 323 ]

17.

DEEP ALTERATION OF CARBONIFEROUS STRATA IN THE MIDLETON, CO. CORK DISTRICT AS DETECTED BY

GRAVITY SURVEYING

BY THOMAS MURPHY, M.R.I.A.

PLATES XXVI-XXVII

[Received 29 MARCH. Read, 14 JUNE, 1965. Published, 22 FEBRUARY, 1966.]

ABSTRACT

AT several places in the Cork Syncline, in particular close to the northern Devonian-Carboniferous contact, closed negative gravity anomalies occur. These range up to 4 mgal amplitude and cover about a square kilometre. The cause suggested is decomposition of the Carboniferous Shale and solutioning of the Carboniferous Limestone to a depth of 180 m below the surface, the cavities being filled with the insoluble residue of the limestone and shale together with other washed-in material. The age, by comparison with similar known occurrences in Ireland and England, is probably Tertiary. The surface of the ground is about 10 m M.S.L. and it is suggested that sea level was more than 150 m below the present level in Tertiary Times.

The infilling may contain minerals of commercial value.

INTRODUCTION

In an earlier paper MURPHY (1962) described numerous instances of small

gravity anomalies. which were thought to be caused by extensive solutioning

of the limestone and subsequent filling with loosely-packed unconsolidated

material. Attempts to verify this supposition were severely hampered by

the occurrence of deep drift which reduced the number of outcrops of rock

in their vicinity to almost zero. Some further work was carried out to obtain

more details of the known anomalies but efforts were made to detect similar

anomalies in areas where the rock was much better exposed. One of these

areas was known to occur in the limestone valley which runs through Cork

City and Midleton to Youghal along the centre of the Cork Syncline

(MuRPHY, 1960). The geology of the district has been fully described by

LAMPLUGH et al (1905), outcrops of rock being plentiful.

SURVEY AND ANALYSIS

Measurements of gravity were taken in the accustomed manner with a

WORD?i gravimeter at stations approximately one mile apart. The positions

are marked on Plate 1. Additional readings were taken in very anomalous

areas so that altogether 260 stations were occupied in the area of 200 sq. miles

i.e. 1-3 station per sq. mile or 1 station per 2 sq. kilometre. The heights and

PROC. R.I.A., VOL. 64, SECT. B. [FF]



324 Proceedings of the Royal Irish Academy.

latitudes of each station were obtained from the six-inch to a mile sheets of the Ordnance Survey as in previous work. The Bouguer anomaly deduced,

using a density of 2167 g/cm3 for the rock layer between the station and mean sea level, is also given in Plate 1.

This plate shows part of the pronounced gravity gradient along the

coastline which is an outstanding feature in this region (MURPHY, 1960). In order to bring out the character of the small anomalies a residual map

has been deduced. This was obtained by calculating the average radial value of the Bouguer field for each station and subtracting this value from

the observed Bouguer value. The radius used was 5 km and the number of points, ten. The method had been used in other districts in Ireland because of its rapidity compared with other methods. Since only adding and

subtracting are involved a small portable adding machine is all that is required and the computation of the disturbing masses is straightforward

compared with, say, the second derivative methods.

The results are shown in Plate 2. Owing to the proximity of the sea

over which no gravity measurements have been taken, the regional values

cannot be obtained out to the coast and hence the map. does not cover the

complete area of Plate 1. The gradient to the SSE has been eliminated.

Presumably this is caused by formations at depths comparable to the 5 km

radius chosen for determination of the regional value.

BOUGUER ANOMALY O- X REGIONAL ANOMALY X ', RESIDUAL ANOMALY '+,'

o i les 3L

O Kglometres 5

10 10

X~~~~~~~~~

A_ _ _ ++ ..........MIDLETON

0 + 0~~~~~~~~~

.A A DEVONIAN C CARBONIFEROUS 1 DEV 1 CARB

2-75 2'l 2-11 2:73 2r 6 2 266

FIGuRE 1-Gravity profiles and geological section along a line A A' on Plate 1. The positions of the sampling sites and the values obtained for the rock densities are given below the geological section.



MURPHY - beep Alteration of Carboniferous Strata in Midleton. 325

The residual anomaly map now exhibits a major negative tract running approximately east-west, flanked on the north by a broad indefinite positive area and on the south by a narrow positive ridge with indications of a further east-west trough on its southern edge. In the main trough, near its northern edge a series of small closed negative anomalies occurs. When this map is

compared with the geological sketch map taken from the one inch to a mile geological maps of the Geological Survey, it is at once apparent that there

is a strong correlation between, on one hand, the positive belts and the

Devonian and on the other between the negative troughs and the

Carboniferous series which here is mainly of limestone. Thus the difference in the level of the anomaly of about 3 mgal, not taking the small intense anomalies into account, must be due to the average difference in density of the two formations.

Profiles of the Bouguer, regional and residual anomalies together with the geological section (from the one inch to a mile maps) and the measured

densities of samples of the rocks are drawn in Figure 1. The line of section,

approximately north-south through Midleton, is given as A A' in Plate 1. It is thought (private communication from D. R. WHITBREAD) that the Carboniferous trough through Midleton is 2-3 miles deep with vertical sides of 15' 175 miles.

Density of the rock formations. From general work in the south of Ireland it is known that the rocks are compact, the pore space being less than

1%, so that the density difference depends entirely on the mineral content of the rock. The Devonian series is for the most part slaty (LAMPLUGIK etc.,

1905) and the sandstones are non-porous. The lower beds of the Carboniferous series are shales and compare in density with the adjacent Devonian. The only light formation of consequence is the Carboniferous Limestone, samples

of which as elsewhere in Ireland give a density of 2-71 g/cm3. The various

densities for water saturated samples are given in Table 1.

TABLE 1-Densities of rock samples

Number

Formation| Type of Density range Average Estimated Mean

Samples Percentage Density

g/Cm3 g/cm3

red slate 20 2%678 - 2-793 2*748 33 1/3

Devonian green slate 7 2.698 - 2-723 2-715 33 1/3 2.716

sandstone 14 2-6.62 - 2-724 2-684 33 1/3

shale 4 2 635 - 2-704 2-659 25

Carboniferous 2-694 limestone 8 2 705 - 2-712 2-708 75

PROC. R.I.A., VOL. 64, SECT. B. [GG]




Of the rock formations given in Table 1, only the limestone can be taken

as homogeneous for thicknesses of the order of a few kilometres. The samples

give densities within a narrow range but it may be thought necessary to

make allowance for solutioning as in the Midleton area many caves are

known to exist. However, from evidence of borings (private communications)

in search of minerals in various parts of Ireland, the Carboniferous Limestone

is in general only subject to solutioning effects within a few hundred metres

of the surface so that it is thought quite safe to accept a density of 2-71 g/cm3

for a formation with a thickness of the order of a kilometre.

In contrast, as shown in Table 1, the densities of the various other rock

types vary from 2 66 g/cm3 for sandstones to almost 2P80 g/cm3 for slates.

The latter are red or purple in colour and probably contain a considerable

amount of haematite.

To arrive at density values for the rock formations that produce the

gravity effects, a division is made between the denser formations, the Devonian

sandstone and slate and the higher formations, the Carboniferous shale and

limestone. Estimates of the percentages, arbitrary in the case of the former and

taken from the cross section of Figure 1 in the latter, are given in Table 1.

The mean densities then become 2 716 and 21694 g/cm3 or in other words a

density difference of 0f022 g/cm3.

This density difference, 0'022 g/cmi, might be thought very small for

computational trials but it cannot be very much greater, certainly not more

than 0'05 g/cm3 for the following reasons. The density measured for the

Carboniferous shale is probably low due to the difficulty in securing

unweathered samples so that 2J70g/cm3 might be a better figure for the

lighter rocks. The value for the denser rocks depends very much on estimating

the relative amounts of slate and sandstone, in effect it is the percentage of

silica that is the important factox. If the denser formation is taken as being

comprised of red slate represented by the 20 samples measured then the

density would be 2 75 g/cinm3 This combined with 2 70 g/cm3 would give

a difference of 0105 g/cmn3 and this then can be looked on as the upper limit.

Computation of gravity differences. It has been lmentioned earlier that, on geological grounds, the 'Carboniferous trough is thought to be about 4km

deep. If, at one extreme, the section is taken to be semicircular with a

diameter of 5 km (cf Plate 2) then with a density difference of 0 022 g/cm3

the anomnaly expected would be -1-4 mgal As the section is thought to

be iectangular in the upper reaches then, at another extreme, taking a

rectangular section 5 knm wide by 4 km deep, the maximum anomaly to be

expected is -213 mgal. Taking the density difference of 0Q05 g/cm3 and the same sections then

the anomalies to be expected would be 3-3 and 5-1 nigal.

As the measured anomaly lies between the computed extremes the gravity

trough running east-west across the residual iilap can be explained by a

small difference in density between the rocks mainly of Carboniferous age



NIiiwivni-Deep Alte,ration of Carboniferons Strata in Midleto a. 327

in the topographic valley and the enclosing formation of Devonian. The

dliflerence in density would lie in the ianrge 0 02 and 0105 g/cm3. A similar

explanation can be given for the gravity trough to the south.

THE INTENSE SMALL ANOMALIES

At several places in the northern gravity trough (Plate 2) the negative

anomaly values are much lower than the -3 mngal discussed above and the

character is completely different so that an analysis on similar lines would

be to no avail.

These anomalies all lie in gravity troughs and in fact on the limestone.

Each anomaly has large gradients associated with it and the areal extent is

small. Because of the latter it was not possible to measure the details without

recourse to levelling which is very time-consuming and not justifiable at

this stage. The lowest value of the anomaly in Plate 2 was measured at a

point one mile west of the village of Carrigtohill which is itself four miles

west of Midletonr The gravity profile tlhrough this point has been drawn

in Figure 2 and this shows gradients in excess of 10 mgal/km and a minimum

value of -7'1 mgal (residual). By comparison the profile through Midleton,

Figure 1, has a low value of -2'8 mgal; a difference of 4'3 mgal. The profile

shows a complicated anomaly and allowing for the effect of the syncline at

least another 4 mgal have to be explained for the deepest part.

SOUGUER ANOMALY O f REGIONAL ANOMALY x 0

RESIDUAL ANOMALY ?' 10 /X10

0~~~~~~~~~~~~~~~~~~~1 ? Kdiometres 5 X

U..~~~~~~~~~~X

+~ ?t

++

+ (0

+ +~~~-1

B B

DEVONIAN 1 CARBONIFEROUS I DEV 1

FiGURRE 2-Gravity profiles and geological sectiorn along a line B B' on

Plate 1



328 Proceedings of the Royal Irish Academy

Further cast, west of Youghal, the gravity troughi is niot as low, being

at about -1 mgal while the smaller anomalies reach -33 mgal, a deficiency

of 2'3 mgal. There are others of the same order elsewhere on Plate 2.

Cause. The size of the anomalies and the gradients involved are similar

to others reported in central Ireland (MURPHY, 1962). For the reasons given

in that account the gravity effects here too cain only be produced by light unconsolidated materials in the limestone. As the water table is close to the

surface only water-saturated material is considered Assuming that these

light materials are sand or boulder clay, the mineral content would have a

density of between 2165 and 2 75 g/cm3 and the void ratio would be

between 0 4 and 0'5. The resulting bulk density of water-saturated material

would lie between 21 and 23 g/cm3 as given in 'Table 2 For the purposes

of calculation the lower value 2 1 g/cmi will be taken

TABLE 2

Computed densities for zvater-saturated unconvoldated sedimentts

likely to occur at Carrzgtohzll

Void Mineral Water-saturated ratio density bulk density

g/cmi g/cm3

0 4 2-65 2 178 1 2*70 2-214

2 75 2*250 0 5 2065 2ll10

2-70 2-133 2-75 2-166

Analysis. The Carrigtohill anomaly has an areal shape approximately

that of a circle one kilometre in diameter (Figure 3). Assuming a cylindrlcal

cavity of this size filled with a material of 2 1 g/cm' the density contrast with

limestone would be 016 g/cm' (2 7 - 24I) The depth of such light material

necessary to produce an anomaly of -4 mgal would be about 180 m (590 f t)

Although 21 g/cm3 can be looked on as a low value even taking densities

as low as 1P8 g/cm3 depths of the order of 100 m are necessary to explain

the gravity deficiency.

Tlohe smaller anomalies west of Youghal have not been surveyed in detail

because of the lack of suitable levelled roads. Whatever form they may take,

the deficiency of mass is equivalent to a thickness of light materials of the

order of 10Gm.

Position. It has already been stated that these anomalies occur in the

limestone valley but so far no outcropping rock of any description has been



MURPHY -Deep Alteration of Carboniferous Strata in Midletonl. 329

.4 -~~~~~~~~~~~~~~~~~~~..

, ?'Z-07 ,.

,* r,, r . . ....... 0 - 3 .. . ;

.~~~~~~~~~~~~~~~~~~~~~~~~~0 .~~~~~~~~~~~~~~.

8 Solid .... . . . . . . . . . . . .

.. .,-,Alvu . . . ,;"@?'s'

Gravel mound __s __________ 1-7 - #hill.t

.. . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~..... . . . . .. ~ ~ ~ ~ ~~~~~~~~~~~.....

N .. . . .. .... ... .. ..~~~~~~~~~~~~~~... ...

~Solid

Re.mainder~ is Bioulder clay with ~~- -

Gravel mrounds 1-7

| Station with BougVer anomaly 2.0.,

FIGURE 3-Position of gravity stations with residual anomaly values together with the drift and solid geology from manuscript maps of the Geological Survey.

found inside the intense anomalous area. This is quite significant and

characteristic of occurrences elsewhere in Ireland. In some places it is possible

that an anomaly occurs in areas mapped as Carhoniferous Shale but this type

of small anomalies has not been detected or suspected outside the Carboniferous

series. When they occur in aieas mapped as Carboniferous Shale then they

lie within a few hundred metres of the mapped limestone contact.

In the Youghal area of anomalies no outcrop of any rock occurs.

The position of the anomalies with respect to the centre of the syncline

is quite significant. They lie in lines near the mnargins of the limestone, the

north side being preferred (Plate 2). It is a distinct characteristic of these

valleys (LAMPLUGH et al, 1905) that the lowest ground occurs immediatelv

at the foot of the steep slopes of the Devonian strata while the centre rises to

kniolls of massive limestone. For example the River Lee at Cork flows along

one side of the limestone syncline.

The ground level at the sites of the anomalies is close to sea level. At

Carrigtohill the highest point is 13 m (40 ft.) M.S.L. atop a gravel mound

and the average height of the gravity stations over the anomnaly is 6-7 m (22 ft.)

M.S.L. The bottom of the light materials as computed above then lies well

below sea level,




Origin. In the earlier paper quoted, the suggestion put forward was

that almost complete solutioning of the limestone had taken place and by

collapse the subsequent cavity was filled with waste material and glacial

gravels. Since that time further work to elicit evidence for this theory has

been collected and forms the subject of another publication.

The main results of this work are that glacial gravels can only be a very

minor constituent if present at all and that the Carboniferous Limestone has

in places suffered extensive deep alteration either by subaerial weathering

or by some other process not yet discovered. The calcium carbonate is

dissolved out and the remaining mainly siliceous part of the limestone is

left along with some washed-in materials, the process having taken place

in preglacial times. A similar suggestion is now offered to explain the origin of the Carrigtohill

and other small anomalies.

One of the reasons for doing gravity work in the Midleton area was also to investigate the possibility of detecting gravity anomalies associated with

the Cloyne Clays. These deposits, reported by BISHoPP and MCOCLUSKEY in 1948, are of silica sands and clays and while their origin was not clear

to these workers it was thought that they were due to the decomposition

of the shales.

Attempts were made to take readings across the belt of the deposits but

because of the lack of levelled roads no measurements were taken at the

actual sites of the occurrences. Nevertheless the readings taken nearby

reveal that there are no anomalies on the scale of one kilometre diameter.

Other readings on the extension of the belt did give anomalies, for example

at the points (1887 640), (1978 677) and (2008 679). Here the geology is hidden by deep drift The size of these anomalies is about -1 5 mgal and

they are not outstanding.

The position of the Carrigtohill anomaly on the northern side of the

syncline very close to the limestone-shale boundary suggests that a similar

decomposition of the limestone shale and the limestone has taken place

but to a much greater degree and extent. The Soil Survey sheets of the

Geological Survey report the whole area as covered by alluvium or drift

mainly of gravel in mounds up to a height of 40 ft. (Figure 3). Directly to

the east, there are limestone outcrops and here the anomaly increases sharply.

On the west at a distance of over a kilometre limestone outcrops. No readings could be taken in this direction on account of the river estuary. A perusal

of aerial photographs (Geological Survey) does not give any indication of

rock close to the surface within the anomaly. It does not seem possible at

present, except at considerable expense, to investigate the matter further

geologically and there are no visual indications which might stimulate

exploration for economic minerals.

Dating. The size and occurrence of the anomalies in limestone invite

comparison with the "Pocket Deposits" of Derbyshire in England (KENT, 1957X



MluI1tuH'v --Deep Alteration of Carboniferous Strata irn Midleton. 331

YOuKEI, 1961) and the glass sands in Wales (THOMAS, 1959). The deposits iti Derbyshire are of sand with clay and pebbles and they, occur in steep

wtllcd cavities in Carboniferous limestone or dolomite. They vary in size, tie largest being of the order of 100 m wide and over half a km long

(YORKB). The depths are not known but they are in excess of 150 m (KENT).

Such deposits should produce anomalies of nearly 2 mgal but so far only

anomalies of the order of 05 mgal have been recorded (Gravity Survey of

Great Britain and private communications from BULLERWELL) and the extent

of the anomalous area is of the order of the deposit. Gravitationally they

are thus not strictly comparable.

The Welsh occurrences are of larger size and anomalies of several mgal

could be expected but no gravity measurements have been taken over them

(cf MURPHY, 1962). There are other occurrences in Ireland and North Wales

but in most cases the dimensions are smaller and usually the depths are

unknown. The Welsh deposits have been given a tentative dating by THOMAS as

"from mid-Tertiary times" while the dating of the Derbyshire ones has been

put as early as Triassic by YORKE who states that they were "trapped in

pre-existing limestone hollows, in part solution cavities and in part great

surface watercourses". The various occurrences of similar but smaller deposits in limestone in

Ireland are being reported elsewhere. They are known to have included

in the in-filling, various materials dated as Cretaceous (WALSH, 1960)' in

Co. Kerry, and Eocene (WATTS, 1957) ih Co. Tipperary. Trhe Cloyne Clay has not been given any date.

It would seem then that the age of the Carrigtohill and Youghal deposits

can only vaguely be put as Tertiary in line with the other deposits thought

to be similar in these islands.

Dsscu SSION

Having arrived at the conclusion that decomposition of the shale anld

solutioning of the limestone have taken place in the Cork-Midleton trough,

it is difficult to explain the process as being confined to deep pits strung out along a line.

Considering first the possibility that subaerial weathering is the cause then

it is to be expected that ground water level would be important. In this

area it is known that the River Lee has cut a trench to 158 ft. below M.S.L.

at Cork City with a gradient of the order of 20 ft per mile. FARRINGTON (1959) while commenting that it is ". . . probably influenced by low sea-level of

a glacial period . . ." does not discuss the dating of the trench further.

At the eastern extremity of the Plates, at the new Youghal bridge, the

River Blackwater has a similar buried valley which ORME (1964) has

estimated to have a base level of -158 ft M.S.L. beyond the present coast.

If the Blackwater at its mouth has a profile similar to the Lee in its lower

reaches, then this estimate of base level would be increased considerably.



332 Proceedtings of the Royal Tiush Academy.

Nevertheless since ground water level must have been about the level

calculated for the bottom of the cavity at Carrigtohill for a considerable time, the corresponding sea level must have been very low at the time for

Carrigtohill is as far from the open sea as Cork City. Furthermore a deep

exit to the sea must also have existed either underground or as a channel.

In the Cork-Youghal limestone valley particularly near Midleton the

limestone is well exposed and while caves etc. are known to exist (LAMPLUGH)

these are small and no gravity anomalies associated with them have been

detected. It is taken that they were produced by subaerial weathering but

at a more recent date. It is not known whether any of these occur below

sea level.

Evidence that solutioning and infilling of Carboniferous Limestone has

taken place to depths at least close to if not below present sea level, has

recently become available. At Drogheda in the limestone quarry of CEMENT

LTD. there occurs a series of deposits in vertical pipes consisting mainly of

silica sand similar in appearance to the Derbyshire deposits. The floor of

the quarry is at 62 ft. M.S.L. and it is obvious that the bottom of the

deposits is well below this level but the depth has as yet not been ascertained.

Four miles to the southwest of this quarry a similar pipe is visible at the

surface at 192i ft. M.S.L. and a boring made in the vicinity has encountered

sand down to a depth of 35 ft M.S.L. without reaching solid rock. This latter

place is six miles west of the Irish Sea. The 35 ft. M.S.L. level is below the depth

of the beds of any rivers in the neighbourhood The filling of these pipes is

not of glacial origin but it contains decalcified chert and remnants of shale

thought to be of Namurian origin (Private communication, WHITEBREAD).

The limestones in the vicinity do not appear to contain chert.

Thus while there is evidence of solutioning having taken place near

Midleton and evidence for cavities in limestone down to present sea level

it is still difficult to visualise subacrial weathering having taken place to

depths of the order calculated at Carrigtohill without very low sea levels in

pre-glacial times. The position of the anomalies close to the Devonian slates appears to

be significant. It may be because the shales were the most easily eroded, as

JUKiES (1862) supposed and the resulting channel would bring the acid waters

coming off the Devonian strata against the adjacent limestone which then

would be attacked more readily (THOMAS, 1954) The depths to which

these waters could reach is again probably determined by ground water

level and so the above remarks regarding very low sea level apply.

On the other hand it is possible that the limestone and shale are attacked

not by subaerial agencies but by some other means for Bisnopp and

MCCLuSKEY (1948) in their investigation of the Cloyne deposits reached

the conclusion that the silica beds were in essence the disintegrated basal

beds of the Carboniferous but confessed themselves baffled as to how this

had come about. They drew attention to their occurrence along with similar

deposits in Ireland, to districts where "there has been appreciable faulting



MuRPHY - Deep Alteration of Carboniferous Strata in Midle ton. 333

and considerable mechanical stress caused by folding and faulting". A suggestion was made:- "The origin of the Cloyne beds is probably bound

up with physico-chemical conditions at the time of their deposition; whether disintegration has been furthered by sulphuric acid liberated by the

weathering of the pyrites which occurs in the underlying Devonian beds is

an open question."

Since the gravity anomalies in the Cork syncline lie in a position

analogous to the silica deposits in the Cloyne syncline it is not unreasonable to

assume that similar conditions prevail but as yet there is a complete absence of any geological evidence. It is thus possible that the light materials present

at Carrigtohill etc. could have a commercial value similar to the Cloyne

and the Derbyshire Pocket Deposits. However, due to their occurrence

close to sea level and the thick drift, their exploitation is another matter.

AcKNOWLEDGMENTS

I wish to thank D. WHITEHEAD of Trinity College for drawing my

attention to the results of the borings carried out by Messrs. ICEMENT Lm.

and J. HUMPHEYS of that Company for acquainting me with the presence

of the solution channels in their quarry at Drogheda. Finally I am grateful

tO CEMENT Lm. for their co-operation and for permitting me to quote details

from their records.

References

BrsHOPP, D. W. and McCluskey, J. A. G. 1948 Sources of industrial

silica in Ireland. Pamphlet 3, Geological Survey of Ireland, Dublin.

Farrington, A. The Lee basin. Part one : Glaciation. Proc Roy. Irish

Acad., 60, B, 135-166,

Jukes, J. B. 1862 On the mode of formation of some of the river-valleys in the south of Ireland. Quart. Journ. Geol. Soc, 18, 378-403.

Kent, P. E. 1957 Triassic relics and the 1,000-foot surface in the southern

Pennines. East Midland Geographer, 8, 3-10.

Lamplugh, G. W. et al. 1905 The geology of the country around Cork

and Cork Harbour. Memoir Geol. Survey of Ireland, Dublin.

Murphy, T. 1960 Gravity anomaly map of Ireland, sheet 5?south west.

Geophys. Bull. Dublin Inst. Adv. Stud., No. 18. - 1962 Some unusual low Bouguer anomalies of small extent

in central Ireland and their connection with geological structure.

Geophys. Prosp., 10, 258-270.

Orme, A. R. 1964 Plantation surfaces in the Drum Hills, County Waterford,

and their wider implications. Irish Geog., 5, 48-72.

Thomas, T. M. 1954 Solution subsidence outliers of Millstone Grit in the

Carboniferous Limestone of the North Crop of the southwest

coalfield. Geol Mag., 91, 220-226




Walsh, P. T. 1960 An occurrence of Cretaceous chalk in the Killarney district, Eire. Proc Geol. Soc No. 1581, 113.

Watts, W. A. 1957 A Tertiary deposit m County Tipperary. Set. Proc.

Roy. Dublin Soc, 27, 309-311.

Yorke, C. 1961 The pocket deposits of Derbyshire. Birkenhead, England,

private publication. Gravity Survey overlay map, sheet 11 1956. Geological Survey of Great

Britain, London.



PRoc. R. I. AcAD., VOL. 64, SECT. B.

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PLATIE 1-Sketch map with National Grid showing the positions of the gravity stations and BOUGUER anomaly contours. The gravity profiles etc. along the lines A A' and B B' are drawn in Figures I and 2.



PLATE XXVI

200 210

10

0~ ~ ~ ~ ~ ~~~~~~8

0~~~~~~~~~~~~~~~~~~

~~~ _ { _ ~~~~~~~~13-_ -.--1_'-YOUGHALS

0~~~~~~~~~~~~~~~~

0~~~~~~~~~1

0 X D o _~ ~~ * N * -bUGE ANOMLY

- 0 |,06g 1 nma

'~~~. -</ / oiino rvt tto

20 . 210

0 B o /0

N I. ~~BOUGUER ANOMALY ) ~~~~~~~in mgal

I Position of gravity station

--- * Jj~. - - National Grid

200 210

(Based on the Ordnance Survey by permission of the Government)



I CY ~ ~ ~ ~ ~ ~~~

ti ~ t

/ I

clJ

I~~~~~~~~~~~~~tn



0

cg

0~~~~~~~~~~~~~

0~~~~

0~~~~~~~~~~

N 10 ~~~~~~~~~~~~ .~~~ L ~ ~ C

cli~~~~~~~~~~~~~~~~~~~U

410~~~~~~~~~~~0

ii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i

________



I,

-s4 ~ ~ ~ ~ ~ ~ ~ -1



q~~~~~~~~~~~~~~~I I~~~~~~~~~~~~~~~~~~b 01

%. .-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i

0 , .

C%j ~ ~ ~

00 ~ ~ ~ ~ ~ I..

I ~~~~~~~~~~~~~~~c

'I~~~~~~~~~~~~~~~~4

Ii *~~~~~~l



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Deep Alteration of Carboniferous Strata in the Midleton, Co. Cork District as Detected by Gravity...

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