DIAMOND: OCCURllENCE IN SOUTH AFRICA 195 



upper part by exposure to atmospheric agencies, and in the lower by the presence of water. 

 Each of the crater-hke basins, or perhaps more coiTectly 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 pulverised and 

 forced up into the pipes by the action of volcanic forces, and, embedded in this eiTipted 

 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 pther 

 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 

 tuft". Since in well-borings 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 

 ihe 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 

 <liamonds, 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 pulverised 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 intact 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 



