612 E. J. DUNN ON THE SOUTH-AFKICAN DIAMOND-FIELDS, 



grey colour and somewhat crystalline texture. It appears to differ 

 from the general mass filling the " pipe " only in "being fine-grained 

 and less liable to decompose. These dykes are from 2 to 3 feet thick ; 

 they cut through the gabbro filling the " pipe," and the carbonaceous 

 shales, grey shales, and dolerite that form the sides of the " pipe." 

 No similar dykes were observed in any of the other mines. 



At Bultfontein mine the shales on the east side are much' disturbed, 

 and are tilted in places at an angle of 60° away from the u pipe." 

 There are numerous blocks and small pieces of dolerite in this 

 mine that have been so rounded by attrition against one another, 

 probably caused by the heaving of the mass when in a molten state, 

 that they resemble boulders and pebbles. These stones are not so 

 noticeable in any of the other mines as they are at Bultfontein. 



As an instance of the value of the ground filling these " pipes," 

 the following (on good authority) is of interest : — 



At Du Toit's Pan 7000 loads, 16 cubic feet each, yielded, on an 

 average, diamonds to the value of £2 12s. per load. 



Discussion 



Prof. Eamsat said the facts mentioned were remarkable ; and it 

 was extremely difficult to say what the circumstances were under 

 which diamonds were developed. 



Mr. J. Evans said that the author had in this paper gone further 

 than he had done in his previous communication. Last year small 

 diamonds had been shown at the Eoyal Society made, it was said, 

 artificially. It would be an experiment worth while for Mr. Hannay 

 to repeat, in the form of heating together pieces of carbonaceous 

 shale and of fusible igneous rock. 



Prof. Seeley said the view of the author was a plausible one. He 

 himself had suggested that carbonic acid might have been carried 

 down by water, and then decomposed by the heat of the volcanos, 

 so that the carbon, when liberated, might become crystallized. The 

 general principle of the author's theory might be true, though, 

 perhaps, not the precise application of it. 



