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aspect, showing intersertal rather than ophitic structure. occurrence of these two slightly differing rock-types may be explained in either of two ways. The doleritic type may form sills intruded into basalt flows in a later phase of activity, as in the Tertiary igneous series of N.W. Europe; or, alternatively, the whole may form parts of one phase, consisting of lava flows of considerable thickness, so that the inner parts of individual flows may have taken on characters resembling those of the hypabyssal rocks.

Mr. H. S. Harger* seems to imply, without explicitly stating the fact, that the whole of this series is intrusive, although he calls some of the rocks basalts, and the petrographical character of the rocks on the whole favours this idea.

So far as can be judged from the specimens in our collection, the general succession of the rocks in this group of mines, viz., Bultfontein, Dutoitspan, and Wesselton, is the same as at Kimberley and De Beers, although, as before stated, the thicknesses are very different. At Dutoitspan the melaphyre appears to be very thin indeed. It would therefore cause needless repetition to describe the specimens in detail.

4. BOULDERS.

Included in the collection are three specimens of boulders, one labelled (409) "Open Mine, Bultfontein," the other two (516) " 470 ft., Dutoitspan.'

One of the latter is of no particular interest, being a very ordinary oligoclase-augite rock, probably belonging to the "melaphyre" series. The other two, however, are worthy of particular description.

The boulder from Bultfontein (409) is of a rounded form, seven or eight inches in diameter, and much decomposed for some distance from the surface. Its central parts, however, are fresh, and show a coarse texture and general greenish colour. The most conspicuous minerals are olivine, green pyroxenes, brown mica and pink garnet. In a slice the dominant constituent is seen to be olivine, which is partly serpentinised, and shows "mesh-structure" in great perfection. The pyroxenes include both rhombic and monoclinic forms, which are fresh and colourless, so that they may be described as enstatite and diopside respectively. There is also a small quantity of grass-green pyroxene, probably the chrome-bearing omphacite referred to by Bonney and others.

The garnets show only the very faintest pink tinge in a good light; they are quite isotropic, and full of the usual irregular cracks. Each is surrounded by a shell of a greenish brown colour, and having a kind of dusty appearance.

Enclosed in this shell, and scattered through the mass of the garnets are abundant small octahedral crystals of a yellowish or reddish brown mineral, of high refringence and isotropic. This is undoubtedly a spinel, and probably one of the chrome-bearing varieties, such as picotite. The shell surrounding the garnets also

* Trans. Geol. Soc. South Africa, VIII., 1905, p. 112.

encloses abundant small crystals of a colourless mineral, probably a pyroxene. These shells are of less regular structure than the socalled kelyphite borders, which are so common in round garnets, but they are undoubtedly of the same nature. The biotite occurs in large plates, and calls for no special remark.

This rock agrees in mineral composition with the special type of enstatite-augite-peridotite, to which the name Lherzolite* is applied, although it contains more garnet than the original rock from the Pyrenees.

The other interesting boulder, from Dutoitspan (516) is of a very different nature. It consists essentially of plagioclase feldspar, a peculiar form of pyroxene, and a good deal of iron ore. The general texture is coarse, but between the large crystals there is a mall quantity of interstitial ground mass, so that, strictly speaking, th rock is porphyritic.

The feldspar builds large, tabular crystals, usually quite idiomorphic, and showing strongly-developed twinning on the albite and pericline laws. The twin lamellæ are broad, as is often the case in basic feldspars. The extinction angles range up to 45°, although the majority are about 30°. Investigation by means of the quartz wedge shows that the vibration direction lying nearest to the twinlamellæ is the axis of greatest elasticity, so that the feldspar belongs to labradorite and bytownite, and not to anorthite, as suggested by its general appearance.

The pyroxene includes two distinct varieties, which are intergrown in the same crystal. One is quite colourless, with oblique extinction at a high angle, as in normal diopside. The other form shows a very good example of diallage structure, having rows of minute inclusions parallel to the basal plane.

Some twinned crystals show the resulting "herring-bone " structure very clearly. As a consequence of this, the hand specimen shows strong schillerization. The diallage has a general yellowish brown colour in a slice, and its extinction is much masked by interference. There is a good deal of iron ore in large, irregular plates.

The small amount of space left between the large idiomorphic crystals is filled by a beautiful eutectic of quartz and feldspar, and in many cases the feldspar of the eutectic can be seen to be in optical continuity with the large crystals.

This rock may perhaps be most satisfactorily described as an augite-porphyrite, although the amount of ground mass is so small that it might be regarded as a gabbro.

IV. SUMMARY AND CONCLUSION.

The general succession of the rocks through which the diamondbearing pipes have been formed, is thus seen to be on the whole very simple. At the lowest level yet reached we have a granite of unknown age. Resting on this is a thin sedimentary bed, and then a great thickness of acid volcanic rocks, which are compared by Rogers

Rosenbusch, Gesteinlehre, p. 173. Harker, Petrology, p. 95.

to the Beer Vley volcanic series.* The overlying quartzites and shales are also considered to be of pre-Cape age; the "melaphyre, which next succeeds, is compared to the Zeekoe Baard amygdaloid, which is also of pre-Cape age. Here there occurs a great gap in the series, since the whole of the Cape formation is absent, and the thin representative of the Dwyka conglomerate, of Lower Karroo age, rest directly on the pre-Cape volcanic rocks. The basalt is in all probability of Karroo age.

The general results of the foregoing petrographical study of the rocks surrounding the diamond-bearing pipes, and the nomenclature adopted may be summarised in the following table, which also gives the maximum and minimum thickness observed in different cases.

[blocks in formation]

The nomenclature here used is purposely left as general as possible, since it is quite impossible in most cases to say definitely from an examination of hand-specimens and slices alone whether a given rock is of a hypabyssal or volcanic nature, and, in point of fact, the distinction between these two classes is a very artificial one. Large masses of lava often assume distinctly hypabyssal characters, on account of somewhat slower cooling, and narrow dykes often consist of rock having typical volcanic structure, e.g., some of the great augite-andesite dykes of the north of England.

The evidence in this case is meagre, and no stress can be placed on it, but it may at any rate be said that none of the igneous rocks seen in these sections, exclusive, of course, of the granite, show any features which are inconsistent with volcanic origin. They can, however, hardly all be part of one great series, since they must cover an enormous interval of time, owing to the great gap in the succession below the Karroo series, if we accept the correlation of these rocks given by Rogers. The acid and intermediate series may be connected although they show no points of resemblance, but the basic series belongs to a much later period.

The diagram on page 288 shows the results arrived at in a graphic form. It is compiled partly from the published works of Mr. Gardner Williams, and partly from information supplied with the collection of specimens.

Geology of Cape Colony, p. 341.

Fig. C.

Diagram showing the succession and thickness of the
rocks passed through in the different Shafts

[blocks in formation]

By F. P. MENNELL, F.G.S.

Curator of the Rhodesia Museum, Bulawayo.

The

The last few years have inaugurated a new era in the history of the South African Diamond deposits, inasmuch as rich bodies of diamantiferous ground have been located at great distances from localities which have previously been worked successfully. group, of which the Premier mine is the best known member, is the most striking example of this extension of area, but the centre of gravity of the diamond-mining industry seems to be gradually shifting northward, and the opening up of the interesting deposit of the Somabula Forest, so far North of any other known occurrence, foreshadows the development of an important branch of the industry in the Cinderella of the South African provinces, as Rhodesia has been not inaptly termed.

No detailed or authentic description of the Somabula field has so far appeared. The writer briefly referred to the occurrence of a remarkably gravelly deposit West of Gwelo in his "Geology of Southern Rhodesia," and ascribed its formation to the Tertiary period, a course which appears fully justified by more recent and detailed investigation. He had already seen diamonds and other gems from the locality, but had not been made aware of their source. Last year he made an examination of the ground on behalf of the South African Option Syndicate, who hold a large area on the field, and who have just erected plant for producing diamonds on a large scale. The reports of their preliminary operations will have shewn that a rich deposit of good quality stones has been opened up. A large quantity were disposed of at a price which works out at £3 178. per carat, and a smaller parcel, sold more recently, fetched £6 per carat. The following notes, for permission to publish which I am indebted to the Syndicate, are intended to give some idea of the geological and mineralogical features of the field, of which I hope a much more detailed account will be given at an early date.

The diamond area may be described as a tongue of the Somabula Forest, stretching along the central plateau of Rhodesia from the Uvungu River for about seven miles in the diretion of Gwelo. The beds of which it consists are undoubtedly younger than the Forest Sandstones, as shewn by their numerous pebbles of agates derived from the lavas interbedded with those rocks, but they are probably not very different in age, and may perhaps be regarded as the uppermost portion of the Forest Sandstone series. They directly overlie the granite of the watershed, on to the apex of which they extend, but further down the Uvungu River the ordinary Forest Sandstones are met with. The general sequence appears to be :

Somabula
Beds

:

(5) Surface rainwash, etc., chiefly redistributed
gravel and sand (often absent)

(4) Red and white sands

(3) Gravel, with partings of clay, etc.
(2) White micaceous sand

resting unconformably upon

(1) Granite.

sav 10 ft.

say 40 ft.

say 40 ft.

30 ft.

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