E. R.
TRANSVAAL
MINES DE.PARTllENT.
REPORT
OF THE
G,I~C)L_OGICAI~ 'S'ITR'VI~Y ~ _J . ____
For the Year 1906.
PRICE 7s. 6d.
PllETO HU :
PUIN'l'ED A'I' 'l'HE GOVEHNi.\IRN'f PHINTING AND S'l'A'rrOKEHY OFI<'ICE.
19m
,. J T 1908 C I h".
CONTENTS.
I.I.---ON TIlE SOUTHERN PORTION OF TI-lE AHEA OCCUPIED BY THE PHETOlUA-]OHANNESBUl{G
GRANITE, by H. Kynasfon, Dil'ectol'.
(Il.~TH1i GEOLOGY OF 'fl-JE l)lSTlUCT ABOUT I-IAENEHTSBUHG, LE'l.'DSDOHP, AND THE lVluHCHISON
RANGE, by E. T. Mellor, Geologist.
I.V.-THE GEOLOGY OF THE CENTl{AL PORTION OF THE MIDDELBUHG DrSTHlCT, INCLUDING THE
TOWN OF MIDDELBURG, by E. T. Mellor, Geologist . •
V.--TIIE GEOLOGY OF TI:IE"CENTRAL POHTIQN OF THE LVDENBURG D~STH.ICT, BET\VEEN LYDENBUl{G
AND BELVEDERE, by A, L. Hall, Geologist,
VI.-ON" POH.TIONS OF TJ-IE LYDENBURG AND CAROLINA DrsTHIcTs, IN THE NEIGHBOURHOOD OF
BELFAST AND MACHADOD.ORP, by W. A. Humphrey, Geologist.
VII.--ON A POHTION OF THE BUSI-IVELD, LYING SOUTH OF THE ROOIBERG AND EAST OF THE Cnoco
. DILE F(IVER,' by W, A, Humphrey, Geologist.
VllI.-THE PRETORIA SERIES' SOUTlI-EAST OF I~USTENBUI~G, by W. A. Humphrey, Geologist.
IX.-THE MAGNESITE DEPOSITS OF MALELANE, by A. L. H"n, Geologist.
X.--~ADDITIONS TO THE MUSEU)\! AND LI13HARY.
v.
THE GEOLOGY OF THE CENTRAL
I~ORTION OF T1--1E LYDENBURG DISTRICT
BETWEEN L YDENBURG AND BELVEDERE.
By A. L. HALL (Ge%.qistj. (
...;.......====. =._._=-' =-=====-'-=-"='--'=""========
V.-THE GEOLOGY OF THE CENTRAL PORTION OF THE LYDENBURG DI~TRICT
BETWEEN LYDENBURG AND BELVEDERE.
By A. L. HALL (Geologist).
r .. TNTRol1nCTION'.
The following report deals with the northerly continlmtion of the fleld work completed during i he winter months of I905.
During that year the mapping was carried as far as a line dmwn from Lydenburg due east to Sabie; in the present year the work was extended in a northerly direction as far as a line dlawn cast :md west through the junction of the Steelport and Olilants Rivers, representing 0. strip of country about .10 miles in leugth ,md 28 miles in width; as before, the base 01 the Black Ieee! Series, roughly coinciding wi\h the edge of the great eastern escarplllent, fonned the limit, t.o which mapping was extended towards Lhe cast; while ou the western side the approxilt,atc bouncl:try was formed by theSpekboom River as far as its confluence with the Steelport Eiver. (Sec Map, Plate XXXIV.).
The area thus exmnined amounts to 1,333 square T\lilcs and contains 1,670 miles of geologica.l houndary lines.
The main statigraphical divisions establisbed for the area further south, described in the Annual Report for 1905, were found to hold good lor the present area also, but it was not always possible to trace out all the leading horiz:ol1s for reasons which will be given below,
There arc many features which make this portion of the Lydenburg District p,rrticularly attractive to the geologist. The scenery may be regarded as perhD.ps the finest in the whole Transvaal, especially Ilear Belvedere, in the neighbourhood of the junction of the Blyde and Ohrigstad Rivers; and where
. the Olifants Eiver cuts through the Black Reef Series, the scenic clements compare favourably with SOme of the best Alpine features for variety and contrast. In the second place, interesting phenomena can be observed ncar the base of the Pretoria Series and of the Dolomite) where mining operations on Frankfort ,md lisbon have afforded exceptionally good exposures; last.J.,)', we have in this district the important gold mining industry of Pilgrims II.est and neighbotlrhood, associated mainly with auriferous horizons ill the Dolomite.
2. PHYSICAL FEATUHES.
In a' district which shows so many different geological formations, ranging from the Magaliesberg 'Iu<trtzite down to the Older Granite, it is not surprising to flnd considerable contrasts in the surface features, and so much is this the case, that it is hardly possible to review thenl {roIn a single standpoint.
With the exception of the tract of country near the junction of the W"terval ,md Spekboom Rivers, the Whole area belongs to the Middleveld type of scenery, although from the point of view of mere <:levalion above sea-level certain parts would he counted as Highveld. It will be convenient to consider the physiography of certain belts of country running roughly north and south, and limited either by main \vater-courses Or by conspicuous t?pographic lines.
(1/) The Valley 0/ the Speilboom and Waterval Rivers. The Spekboom Eiver rises ncar the high ground of shales and quartzites a few miles east of
Lydcnburg and soon after passing this town, if enters country entirely different in charader. This remarkable change can be best studied along the main road connecting Lydenburg with Fort Burger Oil the Steel port H.iver. After crossing the Spekboom River, 3+ miles north of Lydenburg, the road hegins to rise over a wide belt of shales and diabases until onc reaches the long conspicuous ridge, which forms the most noticeable feature in the seener,' west of Lydenburg. This ridge has a north and south COnrs(', for a considerable distance and consists of h<1rdened black shall2s. \,rVhere the road rea~hes the ltighest point-about 620 feet above Lydenburg~"olle obtains a magnificent view towards the north-cast
7G
and east over the undulating table-land composed of the Pretoria Series down to the horizon of the Daspoort quartzite, which liInits the view towards the cast by the. well marked escarpment of the Tunnel quartzite, though from the present position t·he actual escarpment (fac~ng eastwards) is not visible, but only the gentle dip slopes leading ·up to it. Mount Anderson, however, though geologically below the horizon of this quartzite, fonns a conspicuous land-mark.
After crossing this ridge the road immediately descends by means of a series of curves into the narrow ,mel romantic valley of, the \Vaterval River, and the scenery undergoes a complete change. Instead of an open plateau bare of vegetation, one is nOW on, Olifantshoek (1233), some 1,500 feet below the ridge, or about 900 feet b{~low Lyd(-~nhllrg, ill a narrow gorge-like \;alley, del1ned on the west by a striking escarpment of quartzite, and On the east by a dip-slope of a simihr formation, botb belonging to the Magaliesberg horizons. The valley is nchly wooded with a sub-tropical vegetation. From Olifantshoek northwards the ground rapidly falls, so that the junction of the Waterval ,md SpekbooIll Rivers lies at an aprroxiInate elevation of 2)600 feet above sea level or 1,900 feel lower than Lydcnbnrg. At the same time the e~carprnent of IVlagaJiesberg ciuartzite 'continues in the form of a more or less continuous ridge along the western· bank of the vVaterval R:,iver. The main valleys along the lower portion ,of the· vVatcrval and Spckboom I.(ivers have all the chanl.cteristics of Low Veld country.
(b) The C01(,11try between the l3iyde Dnd Speilboom Rivers.
The strip of country confined by these limits comprises the central portion of the area, and its physical features are determined by the presence of a few bands of harder rock, principally quartzites, \vhich offer a much greater resistance to the agents of denudation than the remaining formations. A glance at the map accompanying this report will show the complexity of the boundary lines over this portion, especially near the source of the Ohrigstad and Blyde: Rivers, and when it is remembered that this complexity is almost entirely due to the topography, some idea may te gained of the highly dissected character of the surface.
From the neighbourhood of Waterval Boven northwards as far as the Mauch Berg the hare! quartzites of tbe Daspoort (Tunnel quartzite) and Timeball Hill (Nooitgedacht quartzite) Series practically form a single escarpment, presenting on its eastern.face a steep ~ection, exposing almost the entirecsuccession fl'om the top of the ])aspoort quartzite down to the base of the Pretoria Series. Near the Mauch Berg, hO\:\,cve1', this leadlng topographical feature is divided into two mlnor ones, hardly less marked. . .
·The more westerly of the two extends along the left side of the Ohrigstad Hiver, and forms the escarpment of the middle or Daspoort quartzite, running roughly north and south as far as Ohrigstad, then bending gradually round towards the north-west in conformity with the general geological structure. On the dip-slope side of this escarpment the ground often falls at angles greater than the dip, so that one frequently finds inliers of the quartzite; one of the best illustrations of this can be observed on proceeding_ frOID Krugers Post in a south-easterly direction towards Kranzkloof. As one proceeds northwards, the escarpment gradually approacl~s the Ohrigstacl River, which eventually ncar Ruslplaats cuts into the quartzite parallel to the strike. thus producing a series of more or less isolated and repeatedly notched ftat- topped features, protected from more rapid erosion by circum-eroded cappings of quartzite.
The eastern line of high ground branches off ncar the Mauch Berg, and some 5 miles north of this peak culminates in Mount Anderson (7,489 feet),* composed of diabase, whence it trends northwards along the left side of, and neady parallel to, the Blyde River as far as Kaspers Nek, 9~ miles east of Ohrigstad, From here it shows a somewhat meandering course towards the north-west, similar to that of the western line of hills just described. Occasional ontliers are also Immd in connection with it, as on Haakdoorndraai (447) and near Frankfort (456). The crest-line of this eastern chain practically coincides with the top of the Upper N.ooitgedaebt (Timeball Hill) qnartzite, which thus forms a very striking escarpn1cnt, the eastern face of which has a steep s10pe exposing a continuous succession from the top of the Timeball Hill beds down to an horizon several hUl,drcd feet below the base of the Pretoria Series.
,A fc\y miles north-west of Sabie occurs a short, but sharply defined chain of hills cxt~nding in it
north-easterly direction as far as Burghers Pass, south of Pilgrim~ Il.es\."'" This is due to the lower of the two Nooitgedaeht quartzites, combined with the powerful eroding effects of the headwClters of the Blyde River, which have partially separated it from the 1nain escarpment, The general retarding influence of the harder rock is noticeable even outside the actual limits of its
. outcrop, the higher ground of Dolomite near Burghers Pass being doubtless dne to protection from den]ldation owing to the former presence of this quartzite above it. --.----.-----'"--.---.. -~---.----.. ---
* Height f;upplicd by the Trigonoll1ctl'ieal Survey.
Plate X.
Fig"!. -View' of the Dl'akensbel'g, looking sOllthwat'd h'om "revlls Window," Belvedere,
Fig. 2.---View fI'om Makuks Kraal on Elandsfolltein (167), across the yalley of the Blyde HiYel', west ward towards Maricps Kop, Lydcllbnrg Dist.rict.
·77
From the edge of the main ea~tern ridge a broad dip-slope extends towards the west) and for SOll]('
rlistance from the crest-line one meets an even surface of the typical magnetite quartzite, but tile pririciplc, already mentioned above in connection with the western ridge, is again operative, but to all
even more marked 'extent. The extensive dip-slopes are repeatedly cut into, giving rise to numerous striking kloofs completely encircled by quartzite kranzel1. It is this factor which explains the somewhat peculiar behaviour of the boundary lines over Paardehaal, Breytenbach Kraal (r38), .and Doornhoek, between the Blyde and Ohrigstad Rivers.
No traverse in the whole district brings out these phenomena more forcibly than the main road from Krugers Post over Doornhoek to Pilgrims Rest. Proceeding from' the former point northw8.rds, the route slowly rises until the edge of the western line of hills is reached and the top of tlie Tunnel quartzite, overlooking the valley of the Ohrigstad River. The view northward reveals a series of escarpments and notched table-mountains belonging to the same horizon. The route now winds somewhat abruptly down into the valley, turns sharply to the cast and follows the Ohrigstad II.iver upstream as far as I(osenkrantz. Here begins a long and fairly uniform rise over shales, terminated by a sudden descent over magnetite (Nooitgedacht) quartzite. On Mulder's farm, Doornhoek, this formation gives rise to a roughly circular kranz produced by the erosive action of a tributary of the Ohrigstad I(iver on the dip slope of the eastern chain of hills. Immediately after crossing the stream, the ascent of Pilgrims Hill begins, the summit of which consists of a gently Shelving plateau of the uppermost quartzite of the same (Tirrieball Hill) series. Thenceforth, the road forms a continuous descent, in some parts so steep as to give rise to a gradient of r in 4}, mainly over shales, to the level of the Blyde ](iver, some I.700 feet below the summit. .
Reference may here be made to an interesting effect produced on the topography loy a thick dialoase sheet extending northwards from Krugers Post. Jletween this point and Lyclcnburg igneou;; rocks do not on' the whole have anv marked influence on the scenery, and, when viewed from some distance, appear uniformly merged il;to the general surface. But from ]( rugers Post northwards some of the diabase sheets have a tendency to assume an apparently bedded character, while those, which at the same time possess considerable thickness, give rise to escarpments in the same manner as th~~ ql1art"itc. The best illustration of this feature is seen to the west of the Ohrigstad River. On the map (Plate XXXIV.)., a boundary line.is shown between diabase. on the west and shales on the cast, running ronghly parallcl to the Daspoort quartzite, some two miles west of the Ohrigstad River. In realitv this line forms a very well marked diabase escarpment, as usu81 facing eastwards, and in plaGcs 1,500 feet above thE' level of the Ohrigstad II.iver. Diabasc extends from thc crest-line downwards through a distance of 350 feet, the remainder of the slope consisting of highlv metamorphosed shales, while from the summit ,vestwards one has a dip-slope of diabase, which ascounts for the great areal extent r:1j the
. formation shown on the map. Summarising the physical fcatures of this particular portion of the district, one may conclude
that they are typical of the so-called" Banken " scenery, characterised by a succession of dip-slopes and escarpments, among which those due to the quartzites of the Dasport and Timeball Hill Series and the thick diabase sheet north of Krugers Post stand out more especially conspicuous.
(c) The Country between the Base 01 the Pretoria and Black Reel Series. This portion is limited on the west by thc base of the Pretoria Series, which rarely makes any
topographical fcature, except when the diabase shect, USLEllly found at this horizon, become.s thick enough to give rise to hanzcn. Thc eastern boundary coincides wrth that portion of -the great Drakensbcrg . escarpment, which determines the western limit of the extensive Low Country of this part of the TransvaaL Dolomite and quartzite or sandstones of the Black-Reef Series constitute the leading formations, and the physical features form a marked contrast to those of the adjoining country to the cast and wcst. Since the Dolomite contains no strongly diffe'rentiated horizons, there is an entire absence of. conspicuous topographical lines. From Sabie on the sonth to near Belvedere on the north there is a narrow strip of dolomite, rarely exceeding five or six miles in width, and drained in its southern and central portions by the Blyde Eiver and its numerous tributaries. Further north and north-west, the lower reaches of the Obrigstad River traverse this formation. Nearly everywhere the Dolomite country gives rise to very hilly ground with numerous pointed kopjes of the Spitzkop type, the best examples occurring along the right side of thc Blyde River near the farm Frankfort (456). The main constituting canse of these pecuJi'lrly shaped hills is to be sought for in the pronounced vertical jointing so common in the Dolomite. Sometimes such divlsional planes are repeated in a morc regular manner, and after erosion has accentuated and widened the joints, the scenery recalls that of the Dolomite in the TyroL Looking towards the north-west from the old Waterval Store on Lisbon (No.9), this resemblance is brought ont
78
very forcibly. The gorge-like character of the Blyde River Valley from Pilgrims nest northwards, especially tbe very precipitous walls of dolomite, along which winds the romantic little path to Frankfort, must be attributed to the same causes.
A few miles north of Ohrigstad, where the river of that name passes out of the Pretoria Series into the Dolomite, the country becomes much more wooded and the main valley less confined, the general level is at the same time eonsiderably lower than along the Blyde niver between Frankfort and Pilgrims nest. Hence the topography is not nearly so rugged; the landscape generally has a softer character and more pleasing aspect.
The Dolomite throughout tbe area rests directly on a gently inclined platform of quartzites and sandstones of the Black H.ee! Series, "'Tanged in accordance with the general westerly dip of the Transvaal System. Traced northwards from Sabie Falls this quartzite shelf gradually widens, but never shows any marked hills, only a continuous slightly undulating surface, free from vegetation and watered by many rich streams. North-west of Belvedere the ground is distinctly more hilly. This is not due, however, to the. character of the Black H.eef Series itself, but is caused by outliers of Dolomite, one of which is indicated upon the map (see Plate XXXIV.), south-cast of the junction of tbe Trcuer and the Blyde Wvers:
No mere description can· convey an adequate idea of the grandeur of the country between Belvedere and Blyde River's Poort, where this stream enters the granitic Low-Veld area. The old gold mining camp of Belvedere lies close to the common beacon of the farms 01' de Berg, E.rasmus Hoop, Welgevonden, and Hebron, on an almost level shelf of Black neef shales. The great escarpment of the Drakensberg here forms a fine semi-circular curve, cut into by a nU1l1ber of sprnits, which give rise to precipitous and densely wooded kloofs. Immediately below the edge of this escarpment runs a massive hanz of quartzite, nearly 500 feet in thickness. The rock underlying the camp consists of shales, which fonn the shelf alluded to, while about a quarter of a mile west of the camp rises another eocarpment, the lower portion of which is still b\lilt up of shales and shaly sandstones, capped by the upper series of quartzites. Thus, ncar Belvedere one hao an upper escarpment merging by degrees into a platform of shales terminated eastwards by the main escarpment of the Dmkensberg (see Section, Plate XXV.). Since the junction between the uppermost quartzites and underlying shales forms an important waterbearing stratum, the slopes above the level of the camp arc again densely wooded. The great plain of the Low Country lies some 1,500 feet below Belvedere; and a very steep winding footpath affords the only means of descent. On a clear day the chain of. the Lebombo Hills can be seen on the distant horizon. Since the dip of the Black Eeef Series is towards the west, as one follows the line of the escarpment eastwards, it gradually rises, until it culminates in two superb bluffs 3,500 feet higher than Belvedere, one to the north-east and the other to the south-east (see Plate XL). Simultaneously the shelf of shales becomes more and more reduced in width in these directions, so that eventually it ceases to form any topographical feature. Hence the whole thickness of the Black Reef Series-here some 1,200 feet-takes part .in the formation of the perpendicular walls of bese bluffs.
Traced northwards from Belvedere through Marieps Kop towards the farm Driehoek (548), where the Blyde Hiver has finally reached the Low-Veld, the country occupied by the Black Reef Series presents an even more striking configuration, due mainly to the great thickness of the quartzites, combined with the profound sculpturing action of the larger rivers. From Hermansburg (1035) northwards to the base of the Dolomite the Blyde niver flows in a fairly open valley and is not confined by conspicllouo embankments, but less than half a mile after leaving this formation, the river hao cut its bed, by a rapId succession of cataracts and waterfalls, deeply into the underlying quartzites. Meanwhile the Treuer River and Belvedere Creek, coming from the south-east, have done their share in carving the Black Reef shelf into a striking series of deep winding gorges. They now join with the main stream, and, thus reinforced, the Blyde River flows onwards to erode its channel still deeper into the solid rock, till it runs like a thin silvery streak, at the bottom of a superb canon over 2,000 feet in depth.
Nowhere can these features be studied with greater clearness than near the extreme ea.stern beac'Wof Elandsfontein (167), about a quarter of a mile north of Makuk's Kraal. Plate X., FIg. 2,
illustrates the view. looking eastwards from the kraal, which is built at the edge of the plateau formed by the uppermost beds of the quartzite. From the immediate foreground up to the sky-line the rocks consist of quartzite and shales of the Black Reef Series, the latter, as near Belvedere, occupying an intermediate horizon between the upper and lower quartzites. Between the edge of the main escarpment and the Blyde River, represented in the plate by a short white streak in the centre of the foreground, there is a difference of level of nearly 2,500 feet. The threefold division of the Black Reef Series is clearly indicated in the form of the striking conical kopje seen on the left side of the foreground. It is capped by a . steep cone of quartzite, while about the base there is a kranz of the same rock, the
prate XI.'
POl'tion of the Gl'eat Eastcl'n Escal'pment of the Drakensbel'g, S. of Belvedere . •
7n
gen tIer and well wooded slopes below the upper lU<l!l7- consisting of the middle shale group. The faint sky-line in the centre of the far back-ground is formed by the level plateau of til<> hIstorical Marieps Kop-over 6,000 feet above sea-level, and about 1,800 feet higher than the corresponding plateau in the immediate foreground. Very striking forms of weathering are seen in the massive cylindrical pillars of quartzite surmounted by bowl-shaped or conical cappings of shalcs, the latter being due to the protective influence of a thin remnant of the lower beds of the upper quartzite. If the view is more closely examined, such a case will be ohserved a little to the right of the centre and close to Marieps Kop.
The origin of these striking isolated pillars must be looked for in the numerous vertical joints, which frccly traverse the rocks over considerable distances, combined wIth the powerful process of "deflation," so well described by Walther.* After the beds have first been split up by these vertical joints, dry weathering, acting mainly through the influence of " insolation" (extreme changes of temperature), loosens the small particles of quartz, of which these rocks are composed. Finally, the wind carries the smaller particles away, slowly but surely widening the original joint or cleft, until we have these striking pillars, seen in the photograph.
The same view also shows .the Blyde River emerging from its gorge. A little to the south, the canon assumes its most striking proportions, at the top not more than a quarter of a mile across, at the bOtt01ll only as wide as the river, with a depth of nearly 2,000 feeL . Thus the gorge is on a far grander scale than the better known valley of the Crocodile River at Waterwtl Onder on the Eastern H.ailway Line.
Like the Blyde l{iver,. so the Ohrigstad I~iver presents similar features, when it traverses tbe Black I~eef Series, but they are built On a much smaller scale. From Haakdoorndraai (447) northwards, and ,vhile the river is still on Dolomite, it forms a wide, open and fertile valley; where it enters the underlying quartzites, the valley only becomes slightly narrower and no very striking gorges are to be found. This arises chiefly from the following considerations. Taking the point, where the Blyde River cuts the base of the Do!omite, as 3,500 feet above sea-level, the corresponding point for the Ohrigstad as 2,700 feet, and the junction of both strea!"s as r ,800 feet above sea-level, we obtain the following result :-
Average gradient of Blyde River I in 42. " "of Ohrigstad I in 70.
Thus the Ohrigstad has lost most of its corrosive power much higher up than the Blyde River, and when the latter is still vigorously pursuiilg its course in the bottom of a more or less narrow valley, the Ohrigstad Eiver is meandering sluggishly. A further important factor in this connection is the amount of water in the river. It is well knownt that the corrosive power of a stream increases enormously with its volume. Now, although the Ohrigstad River has a catchment area considerably greater than that of the Blyde, the latter receives far more water,' from its tributaries than the former, so that at anY]loint in its course the volume of water' is always greater in the Blydc River than in the Ohrigstad. The question why the one river receives so much more \vater tha~l the other may be more COIlv(,Jliently discussed in connection with the water supply.
Near Marie]ls Kop the great Drakensberg escarpment gradually bends round towards the northwest. Concurrently with this change in strike there is a marked increase in dip, and hence arises the important transformation in the physical features, so noticeable in the case of the Black I~eef Series between the Blyde and Oliphants I~ivers. From Sabie to Marieps Kop the main escarpment forms practically a continuous line) presenting a vertical face to the Low Country, frDIn which it rises like a great wall. Between these two localities the average dip is between three and four degrees. Hence, leaving out of consideration the sculpturing influences of river erosion, the reverse side of the escarpment .torms a gently inclined and extensive shelf. West of Marieps Kop, however, the dip of the formation varies from 20 to 30 degrees, though the change is not a sudden one. As a result, the face of the escarpment, now looking northward, is no longer vertical, bnt inclined at about 50 to 60 degrees, so that the sheer drops of 2,000 feet and more near Belvedere are gradually replaced by somewhat steep slopes. The best position to study this effect is from the summit of Maricps Kop,:j: \vhere the observer is at a distinctly
• higher elevation than anywhere along the same ridge north-west of the Blyde I~iver. This position also forms the angle of the bend, so tiiat he,.e the contrast between the two types of escarpment is at its maximum.. On Sheet 16 (Selati) of JaCkson's Series of maps, which forms the basis of the geological
"J, Walthcr, <; Oas Gesetz del' \Yiist-enbildullg," Berlin, lHOO, Chapter IV. t See Gilbert., "Geology of t:he Henry ilIoulltaillf>," 'iVaf5hington, 1880, Chapter V. t ThiB eoillcides with the common earner beacoll of Magalics Kop, Glenly.-len, Bedford, Driehock, Hl~'dcl'i\'iers Pom"t., ctc.
80
map on Plate !XXXIV., the difference [between the escarpment south and north-west of Marieps Kop can be appreciated at once from the contouring. Where the Black 1<.ecf Series approaches the Olifants River it forms it chain of conspicuous hills, of which Olihmtsberg* is the most prominent and dist';nctly visible from the valley of the Ohrigstad Wver, a few miles north of the old town. Just as the face of the escarpment has no longer the appearance of a wall, so the reverse side has lost all characteristics of the shelf, which forms such a constant feature of the Black Heef Series further south.
(d) HI ate)' S1.,pPly and Main D)'ainage Lines. On the whole, this part of the Lydenburg district is well watered, especially the eastern portions
over the Dolomite and Black Eeef Series. The main drainage lines are the Blyde, Ohrigstad, and Spekboom Itivers, rising close to Mount Anderson ,rnd eventually flowing into the Olifants ]~iver.
The Blyde H.iver has a length of about 52 miles and a cotclunent area of nearly 300 square miles, extending roughly from the base of the Pretoria Series to the edge of the Drakensberg on the east. This strip of country is well supplied with water, nearly all the streams being perennial. Most of the tributaries joiniug the Blyde Hiver from the east drain the quartzite shelf of the Black Itecf Scriessnch as the Waterfall Itiver, north-east of Pilgrims Itest, the Trener River, Belvedere Creek, and others further north. All the main drainage lines bring rich bodies of very pure water to the main stream, and during the writer's visit in July 'they still carried a considerable quantity. The great eastern escarpment foi'ms a well marked water-shed between the Blyde and Sabie ],ivers; generally the divide coincides accurately with the crest line, a notable exception being found almost due cast of Pilgrims ]'est. Near the south-eastern corner of the farm Lisbon (No.9), the extreme eastern water-shed turns away from the main escarpment in a westerly direction, so that an appreciable area of the lower Dolomite Series drains directly down the Berg into the Low Country by way of the Mac-Mac River.
The history of this stream is somewhat peculiar. Rising near the common boundary of Lisbon (No.9) and Driekop (1350), it flows due south as far as the old river diggings of Mac-Mac on Geelhontboom (220), when it turns abruptly towards the cast and plunges 300 feet down over the edge of the Berg constituting the fmnous Mac-Mac Falls. (Sec Plate XII.). Possibly this is to be accounted for hy a secondary shifting of the drainage lines. On this view the Little Sabie I,iver, now running north and south in a line with the Mac-Mac niver, represents the original course of the latter stream, so that the shurp turn towards the east is the fmal effect. of the cutting back in a westerly direction of a. small tributary rising originally below the escarpment. Hence the present Mac-Mac River would represent a composite river tapped by a. small stream, which at one time had uo connection with it. Throughout the catchment area of the Blyde !\iver we meet with numerous waterfalls, such as those of Mac-Mac, two fine falls of the Waterfall River and its tributaries on Lisbon (9) and Berlyn (458), the falls near the junction of the Treuer with the Blyde Eiver, and others. The view on Plate XII. represents the Mac-Mac Falls looking westwards, which are typical of all these falls, invariably due to some hard slightly inclined bed, usually quartzite.
An important problem in connection with this subject is the steady diminution of the supply of water in the Blyde ]<iver, which is beginning to be felt in the economic requirements of the mining operations ncar Pilgrims I<est. It is true that accurate data extending over long periods arc not available, but the writer is informed that records taken during seyen years show that the diminution now amounts to 38 per cent., and that at the present rate the Blyde River would be dried up in r6 years. If these facts are correct, they undoubtedly open up a serious question, as it is by no means easy to find adequate explanations· for this continuous loss, but it seems possible that some of the water is lost. by percolating westwards through the Dolomite below the Pretoria Series.
The main reason why the hydrographic basin of the Blyde l\iver carries eo much more water than that of the Ohrigstad River is a geological one. Practically no formations other than the Dolomite and Black Reef Series go to make up this basin, and since the quartzites of the latter have the character of sandstones down to some depth the great eastern shelf constitutes a long stretch of permeable ground, over which the Hln-off must be much less than over the shaly regions belonging to the Ohrigstad Hiver basin. The same consideration applies to the Dolomite area. In addition, the position and influence of the great eastern escarpmclit causes an increased amount of precipitation.
The catchment area of the Ohrigstad Hiver amounts to some 512 square miles, while its length measures about 60 miles. The greater portion of this basin from Ohrigstad southwards is composed
~~--.-----.
* This mountnin if; )lot indicatc(1 on .Jackson's mnp, and it seems nncertain how mnch of the hilly ground known locnlly a:o: Olifant!>bcl'g C;lWll\(l be denoted by this tcnn. Its most. conspicuous portion ronghly coineidrs wi! h t·lle H)uth bDnnduries of Calais and Edinburgh,
,I n'/l.llfll Report, Gcolo,qical Sm'I'e!!, 1900.
Plate XI!.
Mac Mac Falls (over Black ncd QUllrt,r.il-c,,), S,B. cf l'ilgrill1s Hest.
81
of the compamtively impenne8.ble shales and diab<lses of tbe Pretoria Series. \Vhile passing through the Dolomite and the Black ned Series, it receives practically no water, so that these ~ormations are Iiydrographically negligeable. The Ohrigstad I{iver has a larger number of tributaries than the Blyde I<iver, but its supply of water is distinctly less. This probably arises mainly from geological reasons, as the formations making up the bulk of the basin are clearly less pervious than the dolomite and sandslones of the Blyde River basin. Thus, with a proportionately greater run-off, most of the precipitation would be at once carried away, and not, as in the case of the eastern basin, largely absorbed, to be returned to the surface in a more' gradual manner, The nature of the beds of the main river and its principal tributaries lend support to this view.
a! the Spekboom l{iver only that portion of its basin on the cast side of the stream comes under the present review: Like the others, it rises from the high ground near Mount Anderson, and, after pursuing a general nor-nor-westerly course, joins the Steelpoort River at Fort Burger. While the main river has a fair supply of water, this is nearly all obtained fro111 the area lying between Krugers Post, Mount Anderson, "-nd Lydenburg; the only tributary of any dimensions is the Waterval Hiver coming from the south-east to join the main stream some 20 miles north of Lydcnburg. Nearly aU the remaining tributaries shown OIl lhe map arc ncglig(!able as regards their supply, for, during the writer's visiL to these parts in the latter half of May, these were already mostly dried up. One finds here, in reality, a waterless strip of country running north and south, and bounded on the west by the Spekboo111 l~iver, from Fort Burger upstream to within some IO miles of Lydenburg, while on be east this arid region is terminated by the striking diabase escarpment beginning at Krugers Post and running northwards along the left side of the Ohrigstad Il.iver. There is, however, every indication of the further extension of this dry region to ncar the junction of the Stcelpoort and Olifants !<ivers. Nearly the whole of this arid strip consists of a sheet of intrusive diabase and intensely metamorphosed shales, possessing a considerable areal extent, but nowhere giving rise to any thick soil. The lower portions of these dry valleys are well wooded, showing that the sub-soil must be retentive to some extent. The appearance of the river bed points to the fact that, at any rate during the short periods of great precipiLttion, the streams must carry a considerable quantity of water, but that this is soon lost owing to the rapid run-off.
, 3. DESCRIPTIVE GEOLOGY.
(a) General Remarks.
The following formations occur III the district M'.lgaliesberg Quartzites Shales with intrusive igneous sheets Daspoort Quartzite Series Shales and intrusive igneous sheets .. Timeball Hill Quartzite Series Shales and In t,:usive I{ocks " Dolomitic Limestone and Chert Thin band of Quartzite Dolomitic Limestone and Chert Quartzi tcs and Sandstones Shaly Sandstones Quartzite and Sandstones
in descending order :..-
I I Pretoria Series
I
I Dolomite Series
I Black Reef Series
Transvaal System. •
Older Granite Swaziland System. The three divisions of the Transvaal System are arranged in sub-parallel belts running north
and south in conformity with the strike, so that the above enumeration represents the natural succession of the va1'i6u5 beds met with in the course of a traverse against the westerly dip, commencing with the lowermost quartzite of the Magaliesberg Series iR the west and ending with the Black Eee! escarpment in the extreme east.
If a line be drawn from Marieps Kop to the junction of the Spekboom and Waterval Rivers, then the strike of the beds south of this line is nearly due north and south, while to the north the strike is towards the north-west and west-narth-west; this change is not sudden but continuous, though somewhat rapid. Careful examination was made to <1sce:-tain whether this structural change Inight be due to repeated step. faulting; but nO evidence was forthcoming to support this v:ew. The only marked c1il1erence between the disposition both north and south of the dividing line is found in the degree of clip) and this only affects tho' Black l\"ccf Series and the lowermost beds of the Dolomite. From Sabic
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