64 Br. 3Iadoth, on the Origin of the [Mar. 31^ 



We know that the diamond fields were once covered with water 

 like the greater part or the whole of South Africa. There is nothing 

 to show that this inundation had come to an end when the volcanic 

 eruption took place which formed these orifices. On the contrary, a 

 great deal of denudation h.is gone on since that catastroi3he happened. 

 I think it, therefore, very probable that the eruption took place during 

 the submersion of the country, and that all these water- worn pebbles 

 and boulders were washed in by the water, either immediately after 

 the first eruption or during the time that the fused mass in the^ 

 orifice was kept boiling. At last the water overpowered the fire 

 and the mass hardened. The immense hydraulic pressure from the 

 overlying water would account at the same time for the highly 

 hydrated state of the diamondiferous mineral, which is therefore 

 neither an igneous nor an aqueous formation, but a child of both fire 

 and water. 



In the light of this theory all the objections I had to raise before, 

 vanish, and I may therefore proceed to deal with the last of the six 

 points mentioned by Mr. Chaper. 



He and a good many other persons were struck by the fact that, as 

 far as each of the four mines extended, a flat hillock rose above the 

 surrounding plain. These hillocks were therefore supposed to be the 

 cones of ejection of the mud-volcanos. I think, however, that the 

 formation of the hillocks is much more recent than the origin of the 

 mines themselves. I take, again, Kimberley Mine as an example, and 

 I may mention for the sake of those who are not sufficiently acquainted 

 with the geological points I have to refer to that a section tlirough 

 the hillock of Kimberley Mine — the Colesberg Kopje — would have 

 shown the following strata : — 



At first a thin layer of red sand, about one foot thick, then a lime- 

 stone conglomerate, several feet thidv, both these layers extending^ 

 equally over the mine and the surrounding country. AVithin the 

 orifice came then the diamondiferous ground, which was j^ellow to a 

 depth of 20 to 30 feet, then becoming blue and much harder. 



The yellow ground is evidently nothing but a decomposed blue, 

 which has changed its colour by the decomposition. Now we 

 know that the blue ground in decomposing increases its volume, and 

 a mining engineer states, that 60 loads of blue give 100 loads of 

 decomposed ground. The proportion is therefore 3 to 5. Let us, then, 

 see what the increased volume of this decomposed blue in Kimberley 

 mine would be. Putting down the surface of the blue equal to n square 

 feet, the original height of the decomposing layer at its average to 25 

 feet, we find its original volume to be w X 25 cubic feet, and its 

 increase after decomposition equal to w X 25 X 5 cubic feet. If 

 this process of increase took place in the open air the mass would have 

 formed a cylinder on the top of the mine. But we know that the 

 mine was covered by a solid layer of lime-conglomerate, and although 

 this crust of lime has been formed in a comparatively recent period, it 

 complicates the question a little. The extending forces in the decom- 

 posing mass could neither succeed towards the bottom nor towards the 

 sides, and on the top they were resisted by this limestone crust. The 

 pressure in the mass must have accumulated, therefore, to a certain 

 extent, and it did so the most in the centre of it. The highest 

 intensity of pressure prevailing in the centre, it is evident that the 

 overlying crust will have been lifted at first on this spot. Once an 



