ORIGINAL COMMUNICATIONS. 



The Original Rock of the South African Diamond. 



By Professor T. G. Bonney, D.Sc, LL.D., V.P.RS. 



In 1867 the first diamond was discovered in South Africa, one having 

 been found in some gravel from the Orange Eiver. Three years 

 afterwards it was obtained in a peculiar deposit of a yellowish colour, 

 like a rotten, rather saponaceous shale, about 15 miles away from 

 the stream and near the present site of Kimberley. There was a rush 

 to the spot, and excavations were soon opened. For some time the 

 mining places were only four in number, and near Kimberley ; a fifth 

 was afterwards added, but all of them lie within a circle of about 

 3-J- miles in diameter. Since then similar deposits have been 

 found elsewhere, and the Newlands Mines, in West Griqualand, to 

 which I shall more especially refer, are about 42 miles to the N.W. 

 of Kimberley. The diamantiferous " yellow ground," as the miners 

 called it, was found, as it was worked downwards, to change gradually 

 into a rather more coherent rock, of a dull dark green-blue colour, 

 named "blue ground"; this became more solid as the workmen followed 

 it downwards, till at a depth of 1200 to 1400 feet it is nearly as 

 consistent as a limestone. 1 In this matrix the diamond occurs, 2 

 together with a number of other minerals, such as garnets (chiefly 

 pyrope), olivine, pyroxenes (including enstatite, chrome-diopside, and 

 smaragdite), a brownish mica passing locally into a chlorite, ilmenite, 

 and magnetite, with small fragments of zircon and kyanite. 3 The 

 ferro-magnesian minerals are more or less serpentinised, and the pyropes 

 are often surrounded by a kelyphite rim, much of it consisting of brown 

 mica. The diamonds, it may be added, are often found, by their 



1 I believe that 1800 feet has been reached in the De Beers Mines, but I have not heard 

 whether the hardness of the rock has materially increased ; probably it has not. 



2 According to the De Beers Consolidated Mines Report, 1889-90, the average yield in 

 that mine is from li to 1J carats per load (about 1600 pounds) ; the Kimberley is much 

 the same. In Bulfontein and Du Toit's Pan it varies from £ to § of a carat per load. 



3 See Lewis, "Genesis and History of the Diamond," for a very full history and account 

 of the minerals, large and small. 



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