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or warted appearance, and may not unfrequently be branched. From these conidiophores the somewhat spindle-shaped spores are given off.
These spores (Figure IV., a., b., and c.) unlike those in the case of the Apple-fungus, never become septate ; and, further, when they germinate, the germ tube arises at right angles to the length of the spore, instead of growing straight out, as in the case of F. dendriticum.
From this examination we are led to conclude that the disease, long known amongst Apples and Pears in Cape Colony, is due to two distinct (but closely related) fungi.
That causing disease amongst Pears is due to the fungus Fusicladium pirinum, Fckl., while that attacking Apples is caused by the fungus Fusicladium dendriticum, Fckl.
These fungi are readily distinguished, the one from the other, under the microscope, and they are identically the same as those which cause the disease known in Europe, Tasmania, Canada, America, and Australia, under various names as Fusicladium, Scab, Scurf, Black Spot, and Cracking.
Fusicladium, Figures I., II., a., b., c. ; III., IV., a., b., c.
ING THE DIAMOND-PIPES OF THE KIMBERLEY
By ROBERT HERON RASTALL, M.A., F.G.S.
Fellow of Christ's College, Cambridge.
(Contributed by Mr. A. F. Williams, General Manager, De Beers
Cons. Mines, Ltd., Kimberley, S.A.)
The manner of occurrence of the diamond bearing rocks of the Kimberley district has been so often described that it is unnecessary to do more than refer very briefly to this part of the subject. As is well known, the diamonds occur in a peculiar brecciated rock, the socalled “ Blue Ground,” which fills a number of vertical pipes or necks, of a somewhat cylindrical form. The mode of origin of these .pipes has given rise to a great deal of controversy, but it is now generally agreed that they are of volcanic origin, although the precise type of vulcanicity which gave rise to them is still undecided. The literature of the diamonds themselves and of the rock which contains them is an extensive one. The whole subject is treated exhaustively in Mr. Gardner Williams's great monograph on the “ Diamond Mines of South Africa,” and in a section contributed by the same authority
Science in South Africa,” the official handbook of the South African meeting of the British Association in 1905.
The object of the present paper is rather to give a short petrographical description of the rocks which surround the diamondbearing pipes, and, in particular, of the rock types which have been exposed in the deep shafts at the Kimberley and De Beers Mines. The former has now reached a depth of 2520 feet, and the latter
The Bultfontein, Dutoitspan and Wesselton Mines are of much less depth, not exceeding 750 feet.
The specimens on which the following descriptions are based have been selected from two collections presented by the De Beers Co. in 1905 to the Mineralogical Departments of the Universities of Oxford and Cambridge. The specimens preserved at Cambridge were placed in my hands for determination by Professor Lewis, and Mr. Hutchinson, and I have to thank Professor Miers, of Oxford, for kindly permitting me to examine certain of the Oxford Series unrepresented in the Cambridge collection. A few of the specimens were collected by Mr. A. Hutchinson, Demonstrator of Mineralogy in the University of Cambridge, during the course of a visit to Kimberley in the autumn of 1905, when facilities for examining the De Beers Mine were afforded him by the Company. In what follows, individual specimens will be referred to by means of the numbers attached to them by the De Beers Company.
Although so much has been written on the diamonds themselves and the rock in which they are contained, there are very few references in the literature to the petrographical character of the rocks
surrounding the diamond pipes. Besides Mr. Gardner Williams' works, already cited, the following may be specially referred to :
BONNEY. On some rock specimens from Kimberley, South Africa. Geol : Mag : 1897, p. 497.
BUTTGENBACH. Quelques observations sur les champs diamantifères de Kimberley. Ann. Soc. Geol : Belgique. Tome XXXII. 1905. Memoires, p. 3.
Rogers. Geology of Cape Colony. 1905. Chapter IX., p. 331.
HATCH AND CORSTORPHINE. Geology of South Africa.
HARGE The Diamond Pipes and Fissures of South Africa. Trans: Geol: Soc: South Africa, VIII. 1905. p.
Numerous further references will be found in the works above cited, and in the catalogue of printed books, etc., relating to the Geology of South Africa, by Miss M. Wilman (Trans: S.A.Phil. Soc.) Vol. XV., part 5.
It is unnecessary to give any detailed account of the superficial deposits of the area surrounding the mines : it must suffice to say that the soil appears to be thin, and is described as of a red colour; it is probably of a lateritic character, derived from the immediately underlying basic igneous rocks. A layer of what is described as limestone is widely distributed over the surface of the district. This appears to be of the nature of travertine or calc-sinter (the calcareous tufa of some authors). At all the mines except Bultfontein, the surface below the soil is composed of a thick bed of a rock, which is commonly spoken of as basalt, varying in thickness from 50 to 100 feet. This is underlaid by some 200 or 250 feet of black carbonaceous shale, and below this again comes a representative of the well-known Dwyka Conglomerate, which is here very thin. These sedimentary rocks scarcely require petrographical description, and no thin slices of them have been prepared.
Below the conglomerate in the Kimberley and De Beers Mines we have about 400 feet of a somewhat decomposed igneous rock, commonly known as melaphyre. Then comes another series of sediments ; 400 feet of quartzite, followed by 260 feet of shale, according to Williams. As we shall see in a subsequent section, this broad division scarcely holds good on close examination, and, indeed, the distinction between shale and quartzite is here purely arbitrary ; both rocks have very similar mineralogical composition, and the differences depend chiefly on the relative sizes of the constituent particles.
At a depth of about 1400 feet in both Mines there begins the great series of acid igneous rocks, the quartz-porphyries of Williams. As we shall see later, this series is by no means uniform, but includes several different petrographical types. At Kimberley this series
extends down to 2470 feet, from which depth comes another specimen of a sediment of somewhat peculiar character. At 2500 feet we reach granite, and the greatest depth represented by the specimens in our possession is 2520 feet. In the De Beers Mine, on the other hand, granite is reached at 1920 feet, so that it appears that the upper surface of the granite is here very uneven, rising 600 feet in about a mile. The presence of a sedimentary rock immediately above the granite suggests that this is a buried land surface, and extreme unevenness is a common character of land surfaces composed of denuded archæan or igneous complexes, e.g., the Lewisian gneiss, and the gneissose area of Brazil, in the neighbourhood of Rio de Janeiro. The other mines, Bultfontein, Wesselton, and Dutoitspan, do not reach a greater depth than 750 feet, so that they throw no light on this question.
In the latter group of Mines the upper part of the series shows some variation in detail, but the general succession is very similar. The differences chiefly occur in the rocks above the melaphyre ; in the diagram given by Williams, quartzite is shown both at Bultfontein and Dutoitspan at a much higher horizon than elsewhere. We shall return to this question later on.
Since the Kimberley Mine is the one which has up to the present reached the greatest depth, it may for our present purpose be taken as typical, and will be described first, and in considerable detail. Some of the rock types, however, show better development elsewhere; in such cases description will be deferred till the mine in question is treated of.
Since the ground surface around all the mines is at practically the same height above sea level, varying only a few feet on either side of the contour of 3990 feet, it will be convenient in all cases to use the surface as a datum line, and to speak always of depths below this generalised surface. Since the depths given appear to be approximate, correct to only 10 feet or so, no appreciable error will be introduced, and the descriptions will be more intelligible than if heights above sea-level were used.
I. KIMBERLEY MINE.
The present surface of the ground at the Kimberley Mine consists of material which is collectively described as debris, having a total thickness of 30 feet, overlying 5 feet of red soil, which is probably of a lateritic nature. Below this we come to the usual basalt, which is here 50 feet thick, rather below the average. The specimens of basalt from this locality (201) (a) are much decomposed, while excellently fresh examples of what is undoubtedly the same rock come from other mines, and notably from Wesselton, so that consideration
(a) Numbers in brackets refer throughout to the original numbering of the
specimens as supplied by the Company.
of this rock type may profitably be deferred for the present. Under-
Below the conglomerate we come to the next great development of
The dominant minerals are a plagioclase feldspar and green chloritic pseudomorphs, representing some member of the ferromagnesian group
The feldspar occurs in idiomorphic, somewhat elongated prisms. The majority show twinning on the albite law, and often Carlsbad twinning also. The extinction angle, measured on the albite twinlamellae, rarely exceeds 5°, so that the feldspar may be regarded as oligoclase. In a few cases a somewhat higher angle indicates andesine, a few sections show Carlsbad twinning only, and these prisms are rather shorter and stouter than the others, so that a little orthoclase may be present. This point is somewhat doubtful.
The ferromagnesian minerals are unfortunately very much decomposed, being chiefly represented by green chloritic pseudomorphs. Many of these pseudomorphs enclose, however, a few fibres of a pale or colourless actinolitic hornblende, of the character usual in this mineral when of secondary origin, from the uralitisation of augite, and a few crystals of comparatively unaltered augite may also be
It it clear, therefore, that the original mineral was augite, which has undergone the usual cycle of changes, being first converted by uralitisation to actinolitic hornblende, and this in its turn to chlorite.
Some specimens of the rock show a certain amount of interstitial quartz, and in places it is clear that the last substance to crystallise was a eutectic of quartz and feldspar, as is so common in the more acid members of the dolerite group.
There is a good deal of iron ore in small scattered grains, including both magnetite (or ilmenite) and pyrites, and apatite is abundant in minute needles. There are also a certain number of idiomorphic crystals of a pleochroic yellowish green epidote, which may be an original constituent, but is more probably secondary. As
(b) Rogers. Geology of Cape Colony, p. 147.