In 1874 an account of the chemical and optical properties of the yellow and blue ground, and the contained minerals was given in London to the members of one of the learned associations there by Prof. N. Story-Maskelyne, F. R. S., keeper, and the late Dr. W. Flight, assistant, of the mineral department, British Museum, and although rather technical in character will no doubt be interesting to those who would like to make themselves acquainted with the formation of the diamond mines. Extracts from this account will show the nature of the rock commonly but erroneously called the matrix of the diamond, and also furnish a list of minerals found in the samples furnished to the above mentioned scientists. Alluding to the yellow and blue these gentlemen say:
“Ground mass, pulverulent, soapy, light yellowish in the upper, and of an olive-green to bluish gray color in the lower regions of the excavations, is a hydrated bronzite. Through it is disseminated a considerable amount of vermiculite.... In this ground mass fragments of shale and a micaceous looking mineral—sometimes an important constituent.... It is a mineral of the vermiculite group.... Ferriferous eustatite (bronzite), prismatic crystals of a bright green color not infrequent, of the size of canary seeds—colored brilliant green by nickel.... Hornblendic mineral accidental—closely resembling smaragdite garnet. Ilmenite.[[26]]... Diallage much altered, opaline silica, sometimes resembling hornstone. Calcite. Bronzite, small bright green crystals, with something of an emerald tint, prismatic; angle 87° 20′. Hydrated bronzite; drab (or pale buff tint) much broken up, and cemented by calcite in bar-like forms, resembling feldspar.... Through it is disseminated a considerable amount of vermiculite—vaalite—surface of a fine bluish green like that of clinochlore, giving color to the mass. Hexagonal prisms 60° and 120°, resembling halite. Garnet zircon, brownish white. Hornblende crystals, with the appearance of smaragdite. Grossular garnet. Brilliant little black tourmalines. Smaragdite:—brilliant grayish green fragments of crystal, angle 125° 15′ equal to hornblende type. Olivine, steatite; variety of vermiculite, or a ferriferous eustatite or bronzite. Transparent striated mineral, of fibrous irregular outline, augitic cleavage; pale brown, in some lights a violet tinge. Between the fibres minute bars of brown vaalite, but tolerably free from calcite; occasionally associated with a yellow wax-like substance, probably opal. This striated mineral may be regarded as made up of
| Bronzite | 43.850 |
| Hydrated bronzite | 24.017 |
| Opaline silica | 30.895 |
| Alumina | .970 |
| Chromium oxide | .251 |
| 99.983 |
“The base of the rock[[27]] consists of the same ingredients as that described above, the mass of it being the hydrated bronzite. The rock is further very full of fragments of the shale, which has been altered, but still contains carbon; indeed the character of the rock is almost that of a breccia, in which these masses of shale are cemented by hydrated bronzite containing the vaalite and the bright green bronzite, with ilmenite and the other minerals associated with it.... The several minerals composing the rocks, exhibit this undoubtedly once igneous rock in the light of a bronzite rock, converted into a magnesium silicate, which has the chemical character of a hydrated bronzite.... The steatite like magma in which the other minerals and shale fragments are contained may have originated in an augitic mineral, but this is not very probable. The alterations that have ensued from the shattering of the eustatite rock, at a period subsequent to its becoming solidified, having aided in effecting the hydration that has so largely changed it from an eustatite rock into a mixture of eustatite with a hydrated eustatite, a combination which, both in its composition and structure, recalls vividly to the mind the similar mixture of the former mineral with the so-called pseudophite in which it occurs at Zdar, in Moravia.”
From the foregoing description a fair idea may be arrived at of both the formation and appearance of this mine, which may fairly lay claim to be one of the wonders of the world. In the next chapter I will give the principal theories concerning the origin of this mine.
CHAPTER XI.
SOURCE OF DIAMONDIFEROUS SOIL.—EXTRACT FROM MESSRS. BAIN, URE, DANA AND OSBORNE’S GEOLOGICAL THESES.—VARIOUS THEORIES ADVANCED—NONE ENTIRELY SATISFACTORY OR CONCLUSIVE.—THE ORIGIN OF THE PRECIOUS STONE VEILED IN MYSTERY.
The extracts in the previous chapter, taken from the papers of the afore-mentioned mineralogists, show, there can be no doubt, that the diamondiferous soil of the diamond mines of South Africa is, for the most part, the débris of an igneous rock; but little or no idea is given how the mines became filled with the soil. Although I am unable to throw any additional light upon this very difficult subject, I nevertheless am of opinion that this chapter would be incomplete without giving a few more extracts from previous writers. I may mention, however, that scarcely a sufficient number of facts have as yet been gathered to enable geologists to offer any theory of the formation of the diamond mines that will carry conviction with it.
In a report to Col. Charles Warren, acting administrator of Griqualand West, by Mr. T. C. Kitto, a mining engineer who was then visiting the province, and who previously had some experience in Brazil, and which report was published in the local government Gazette of July, 1879, he states: “I shall at once assume the Kimberley mine formation to be the result of earthquakes and volcanic agency. That the De Beers, Du Toit’s Pan, and Bulfontein belong to the same group; that the diamond deposit has been ejected from below, and that the diamonds were formed previous to their final deposition in the crater.”
Another geologist, Mr. Thos. Bain, district inspector P. W. D., “considers the numerous superficial deposits of calcareous tufa are the detritus of the tertiary deposits,” as the following quotation from Silver’s “Hand Book of South Africa” suffices to show: “In reference to the beds of clay-stone porphyry before mentioned, Mr. Bain supposes them to be the products of a vast volcano situated somewhere in the Drakensberg range, whose products spread ruin and desolation over the carboniferous forests for hundreds and thousands of square miles, and were afterward swept away by the action of water, except what yet remains of the débris in those porphyry dykes, and the greenstone tops of the multitudinous hillocks and kopjes in the region toward the north. The elevated plateaus of Hantam, Roggeveldt, Nieuweld and Sneeuwberg form its inland boundaries. This immense desert, as geology tells us, was once a great lake, bordered by an umbrageous flora, whose former existence can only now be attested by the petrified monocotyledons buried in its finely laminated slates, and whose waters were crowded with the numerous adentulous animals or the varied family of dicynodons and other saurian reptiles found in no other part of the globe.”