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Dry Diggings (suite)

He regards the pipes as volcanic vents or chimneys comparable with those, also extinct, of the Eifel, and considers that the serpentine breccia now filling the pipes was brought up from below by the action of volcanic forces, but at what period of geological history this took place neither he nor later authorities can say.

To quote Cohen's own words:
"I consider," he says, "that the diamantiferous ground is a product of volcanic action, and was probably erupted at a comparatively low temperature in the form of an ash saturated with water and comparable to the material ejected by a mud volcano. Subsequently new minerals were formed in the mass, consequent on alterations induced in the upper part by exposure to atmospheric agencies, and in the lower by the presence of water. Each of the crater-like basins, or perhaps more correctly funnels, in which alone diamonds are now found, was at one time the outlet of an active volcano which became filled up, partly with the products of eruption and partly with ejected material which fell back from the sides of the crater intermingled with various foreign substances, such as small pebbles and organic remains of local origin, all of which became embedded in the volcanic tuff. The substance of the tuft was probably mainly derived from deep-seated crystalline rocks, of which isolated remains are now to be found, and which are similar to those, which now crop out at the surface, only at a considerable distance from the diamond-fields. These crystalline rocks, in which the diamonds probably took their origin, were pulverized and forced up into the pipes by the action of volcanic forces, and, embedded in this erupted material, these same diamonds, either in perfect crystals or in broken fragments, are now found. Analogous cases of the simultaneous ejection of broken and of perfect crystals are afforded by some of the active volcanoes of the present day, and moreover, in many other localities, the mother-rock of the diamond is probably to be found in the older crystalline rocks. At any rate, these rocks contain, as a rule, just those minerals, which are most frequently associated with diamond. The beds of shale and sandstone interbedded with sheets of diabase were broken through and fractured by the force of the eruption, and so large blocks (floating reefs) and small fragments of these rocks became embedded in the tuff. Since in wells bored in the neighbourhood of the mines bands of coal are often met with interbedded with the shales, the coal, which is occasionally found in the diamond-bearing ground, and which has been incorrectly thought to have some genetic relation with the diamond, must have been derived from the seams of coal interbedded with the shales".

The fact that there is no genetic relation between the coal found in the tuff and the diamonds, or in other words that the diamonds have not been formed in the pipes from fragments of coal, is clearly shown by the frequent occurrence of diamond crystals in broken fragments. Had the diamonds been actually formed in the "blue ground", it would be difficult to find any explanation of the occurrence of so many broken crystals. If, on the other hand, we suppose them to have been formed in a deep-seated crystalline rock, which by the action of powerful volcanic forces was pulverized and forced up into the pipe or funnel, the fragmentation of many of the crystals follows as a matter of course.

That the material filling the pipes was not washed into them by flowing water is proved by the absence of any trace of wear in the minerals and rock-fragments enclosed in the tuft; all of which preserve in fact the sharpness of their edges and corners. Had the soft and fragile materials, such as the abundantly occurring shales and mica, been transported by water over even the shortest distance, they would inevitably show some sign of their journeying.

A volcanic origin for the diamantiferous deposit thus appears to be the only possible conclusion, which can be drawn from the observed facts; it should be noted, however, that according to this theory the diamond itself did not originate in the same way, but was formed in a deep-seated rock before the eruption took place. Cohen's theory is so closely in agreement with the observed facts that it has been very generally accepted, and up to the present has only required modification in a single particular. In this theory it is assumed that each pipe was formed and filled up by a single manifestation of volcanic activity and that, excluding of course the effects of subsequent weathering and alteration, the pipe as we now see it is the product of this single eruption. A consideration of the vertical columns into which the pipes of "blue ground" are divided, and which differ from each other in such characters as colour, composition, contained minerals and richness in diamonds, has led Chaper to the conclusion that each of these vertical columns is the product of a distinct eruption. Since the columns are similar in general character and differ only in minute details, he considers that they have been formed by a series of eruptions of the same type; in short, that each diamantiferous deposit or pipe is the product of a long-continued period of volcanic activity. Thus, according to Chaper's view, the pipe of the Kimberley mine, in which fifteen columns have been observed, is the result of fifteen successive eruptions. Further observations in this direction are, however, desirable.

From the considerations brought forward above, it seems very probable that the South African diamonds were formed in a deep-seated crystalline rock, which became fragmented and erupted to the surface by the action of volcanic forces; and moreover, that the greater part of this ejected, fragmentary material fell back again into, and filled up the vent or crater produced by the eruption. From the nature of the minerals, which accompany the diamonds in the volcanic tuff, it is perhaps possible to draw some conclusions as to the character of the rock in which the diamonds were formed. Almost all the minerals, which are constituents of the rocks generally known as olivine-rocks, and which are widely distributed in the earth's crust, are found amongst the minerals associated with the diamond in "blue ground. " It is therefore highly probable that the original mother-rock of Cape diamonds was an olivine-rock, situated at a great depth below the earth's surface and containing as constituents biotite (represented by the altered mica, vaalite, enstatite (bronzite), garnet, and all the other minerals already mentioned, including of course the diamond. Such a rock, which would be similar to an lherzolite in composition, has indeed, though in a somewhat different sense, been named kimberlite, and from this, the "blue ground" filling the pipes has been referred to as a kimberlite-breccia or a kimberlite-tuff. This kimberlite-breccia or tuft, at least as far down as it has been reached by mining operations, has undergone great alteration, its originally predominant constituent olivine being almost completely altered to serpentine, so that very little of it is now to be seen. This more or less complete alteration of olivine to serpentine in an olivine-rock is not at all unusual, being a matter of common observation in all parts of the world. The other constituents of the original rock have undergone less alteration and are in a more or less fresh condition. At greater depths the decomposition of the olivine has been less complete, and here may be found traces of kimberlite still unaltered, in which the olivine retains more or less completely its original character.