Incwadi Yami; or, twenty years' personal experience in South Africa

by J. W. Matthews

The first rock under the black shale gone through, as shown in the diagram, is a light gray volcanic rock, three feet thick, with a specific gravity of 2.815. The body of the rock is of a light bluish gray color, and contains beside other minerals nodules of quartz, agates and jasper, from the size of a mustard seed to that of a walnut.

Next is found eight feet of a compact augitic or hornblendic rock, of a deeper gray color and very tough, then a seamy rock resembling basaltic trap, which was only worked to a depth of four feet in the shaft shown in the section, but is of unknown depth, for, since the Mining Board shaft was abandoned, others have been sunk, one by the French company, and another to a depth of 620 feet by the Central company, and they both are still in this same rock.

This rock contains very numerous nodules of quartz (amygdaloidal), some of which are split into flakes, and others completely fractured. Many of these are white in color and semi-opaque; others have a red skin (jasper), with transparent white quartz within, while others are entirely red. There are also many small, dead grain-like nodules of a white color, as well as ribbon and other agates. In some of the deeper portions of this rock are many small fissures, presenting somewhat the appearance of having been caused by shrinkage; these, in most cases, are distinct from each other, and are filled with white crystallized calcite, or carbonate of lime. Some of the layers of this rock vary in color, and a very compact fine-grained specimen of a reddish color contains amongst other minerals rhombic crystallized carbonate of lime, white; very brilliant iron pyrites, and a mineral looking very much like galena, but harder, of a dark steel gray color, very brilliant, and easily separated into small cubes and laminæ. This is probably specular iron ore. Another mineral has also been found in small quantities, running in veins in the stone, which upon the top appears as having been fused and run, like tin or lead. It is sectile, and of the color of pale bell metal. Very many forms of crystallized carbonate of lime, including “dog-tooth” spar of a pale yellow color are found both in the hard and soft reef, and zeolites of various colors and species, bristling upon and within cavities of the rock. The reef as a whole is fairly even all around the mine, but at the southeast it is very much contorted in the upper layers.

The wall formed by the hard rock around the mine is very compact and smooth, and runs inward in some places as much as 30° or 40°, but in others very much less. This naturally causes a cutting out of the claims or a contraction of the mine, but it is said that the blue ground of the Kimberley Central company is now gaining again, and this state of things may also take place with other claims in the Kimberley mine at different depths. (See illustration.)

I think this is a fairly full and accurate description of the geological strata outside of the Kimberley mine. I shall now pass on and give a description of the formation of the interior of the mine.

When the place was first prospected there was little to distinguish it from the surrounding country, but in the eyes of the experts of those days the slightest difference was enough to urge them to seek, “fossick,” or prospect for diamonds, that is to say, scratch or dig up the surface, sieve and sort it, and sink small trial shafts, when if diamonds or good indications were not discovered at a moderate depth, the place was abandoned, and the prospector tried or “fossicked” elsewhere. A digger would pitch upon what appeared to him to be a likely spot, when, if after passing the red sand and the lime or calcareous tufa, he came to shale, he abandoned his “prospect” as useless; but if, on the other hand, after going below the lime he came to “yellow ground,” a substance something like greenish compact wood ashes, he would continue his work for some time, in the full expectation of being rewarded in the end by a good find. But in many cases this desired result was not attained, although the digger had every encouragement to persevere by finding garnets of various kinds, the pyrope garnet, usually called a ruby, especially giving him encouragement to proceed with his work; epidote, pisolites, talc ilmenite, called by the diggers carbon, iron pyrites, ice spar, zircon, and various other minerals; still after all his perseverance, and after finding all these indications, and sinking to a depth very often of twenty or thirty feet, not a single diamond would be found, he would abandon the place, when possibly after a time it would be tried again by other parties of diggers, but with no better result.

The surface, as already stated in the description of the reef, was a red, sandy soil of an almost uniform depth of six feet, followed by a layer of calcareous tufa and one of yellow diamondiferous soil, averaging in thickness sixty-five feet.

In the red sand diamonds were frequently found, especially when it was mixed with nodules of calcareous tufa which had been thrown out by the ant-bear (Myrmecophaga jubata) in making its enormous burrow, but the distribution of the diamonds which were picked up on the surface of a large tract of country was mostly attributed to birds. Coming next to the tufa bed proper, this was of a thickness from two to eight feet, but was not a compact homogeneous mass, but composed of honeycombed nodules and masses impacted together, which required much labor to break out. Some diggers smashed up all the nodules thus broken out with sledge hammers, or the sides of their picks, and sometimes, but very rarely, they were rewarded by finding diamonds.

When the yellow ground was arrived at, which, as already stated, was of an average depth of sixty-five feet, and contained many nodules of calcareous tufa of all sizes, the color was of a pale yellowish green, but when it became dry it was somewhat of the color of bath-brick, very friable, and appeared much like fine wood-ash pressed together. It could be broken up to a fine powder with very little beating either with sticks or shovels. Most of this yellow ground was sorted dry, as washing was not practiced at the dry diggings at that time.