346 



SCIENCE. 



LVoL. Vm., No. 193 



of rolled pebbles gathered in the Orange River. 

 This led to the ' river-diggings ' on the Orange and 

 Vaal rivers, which continue to the present time. 



In 1870, at which time some ten thousand persons 

 had gathered along the banks of the Vaal, the news 

 came of the discovery of diamonds at a point some 

 fifteen miles away from the river, where the town of 

 Kimberley now stands. These were the so-called 

 ' dry diggings,' at first thought to be alluvial depos- 

 its, but now proved to be volcanic pipes of a highly 

 interesting character. Four of these pipes or necks, 

 all rich in diamonds, and of similar geological struc- 

 ture, were found close together. They have been 

 proved to go down vertically to an unknown depth, 

 penetrating the surrounding strata. 



The diamond-bearing material at first excavated 

 was a crumbling yellowish earth, which, at a depth 

 of about fifty feet, became harder and darker, finally 

 acquiring a slaty blue or dark green color and a 

 greasy feel, resembling certain varieties of serpen- 

 tine. This is the well-known ' blue ground ' of the 

 diamond miners. 



It is exposed to the sun for a short time, when it 

 readily disintegrates, and is then washed for its 

 diamonds. This blue ground has now been pene- 

 trated to a depth of six hundred feet, and is found 

 to become harder and more rock like as the depth 

 increases. 



Quite recently, both in the Kimberley and DeBeers 

 mines, the remarkable rock has been reached which 

 forms the subject of the present paper. The geo- 

 logical structure of the district, and the mode of 

 occurrence of the diamond, have been well described 

 by several observers. 



As Griesbach, Stow, Shaw. Rupert Jones, and 

 others have shown, the diamond-bearing pipes pene- 

 trate strata of carboniferous and triassic age, the lat- 

 ter being known as the Karoo formation. 



The Karoo beds contain numerous interstratified 

 sheets of dolerite and melaphyr, also of triassic age, 

 the whole reposing upon ancient mica schists and 

 granites. The careful investigations of Mr. E. J. 

 Dunn demonstrate that the diamond-bearing pipes 

 enclose fragments of all these rocks, which frag- 

 ments show signs of alteration by heat. Where the 

 pipes adjoin the Karoo shales, the latter are bent 

 sharply upwards, and tho evidence is complete that 

 the diamond-bearing rock is of volcanic origin and 

 of post-triassic age. 



The diamonds in each of the four pipes have distinc- 

 tive characters of their own, and are remarkable for 

 the sharpness of their crystalline form (octahedrons 

 and dodecahedrons), and for the absence of any signs 

 of attrition. These facts, taken in connection with 

 the character of the blue ground, indicate, as Mr. 

 Dunn has pointed out, that the latter is the original 

 matrix of the diamond. 



Maskelyne and Flight have studied the microscopi- 

 cal and chemical characters of the blue ground, and 

 have shown that it is aserpentinic substance contain- 

 ing bronzite, ilmenite, garnet, diallage, and vaalite 

 (an altered mica), and is probably an altered igneous 

 rock, the decomposed character of the material ex- 

 amined preventing exact determination of its nature. 

 They showed that the diamonds were marked by etch 

 figures analogous to those which Prof. Gustav Rose 

 had produced by the incipient combustion of diamonds, 

 and that the blue ground was essentially a silicate of 

 magnesium impregnated with carbonates. 



The blue ground often contains such numerous 



fragments of carbonaceous shale as to resemble a 

 breccia. Recent excavations have shown that large 

 quantities of this shale surround the mines, and 

 that they are so lyghly carbonaceous as to bn com- 

 bustible, smouldering for long periods when acci- 

 dentally fired. Mr. Pater son states that it is at the 

 outer portions of the pipes where the blue ground is 

 most heavily charged with carbonaceous shale that 

 there is the richest yield of diamonds. 



Mr. Dunn regards the blue ground as a decomposed 

 gabbro, while Mr. Hudleston, Mr. Rupert Jones, and 

 Mr. Davies regard it as a sort of volcanic mud. Mr. 

 Hudleston considers that the action was hydrother- 

 mal rather than igneous, the diamonds being the re- 

 sult of the contact of steam and magnesian mud 

 under pressure upon the carbonaceous shales, and 

 likens the rock to a ' boiled plum-pudding.' 



The earlier theories as to the origin of the diamond 

 have, in the light of new facts, quite given way to 

 the theory that the diamonds were formed in the 

 matrix in which they lie, and that the matrix is 

 in some vvay of volcanic origin, either in the form of 

 mud, ashes, oi lava. 



The exact nature of this matrix becomes, there- 

 fore, a matter of great interest. The rocks now to 

 be described are from the deeper portions of the 

 DeBeers mine, and were obtained through the 

 courtesy of Mr. Hedley. They are quite fresh, and 

 less decomposed than any previously examined. 

 Two varieties occur, — the one a diamantiferous, the 

 other free from diamonds, — and the lithological dis- 

 tinction between them is suggestive. The diamantif- 

 erous variety is crowded with included fragments of 

 carbonaceous shale, while . the non-diamantiferous 

 variety is apparently free from all inclusions, and is 

 a typical volcanic rock. 



Both are dark, heavy, basic rocks, composed essen- 

 tially of olivine, and belong to the group of perido- 

 tites. Both are of similar structure and composition, 

 differing only in the presence or absence of inclu- 

 sions. The rock consists mainly of olivine crystals 

 lying porphyritically in a serpentinic ground-mass. 



The olivine is remarkably fresh, and occurs in crys- 

 tals which are generally rounded by subsequent 

 corrosion. The principal accessory minerals are 

 biotite and enstatite. The biotite is in crystals, often 

 more or less rounded, and sometimes surrounded by 

 a thin black rim, due to corrosion. Similar black 

 rims surround biotite in many basalts. Tho biotite 

 crystals are usually twinned according to the base. 

 The enstatite is clear and non-pleochroic. Garnet 

 and ilmenite also occur, the latter often partly altered 

 to leucoxene. All these minerals lie in the serpen- 

 tinic base, originally olivine. This rock appears to 

 differ from any heretofore known, and may be de- 

 scribed as a saxonite porphyry. 



The diamond-bearing portions often contain so 

 many inclusions of shale as to resemble a breccia, 

 and thus the lava passes by degrees into tuff or vol- 

 canic ash, which is also rich in diamonds, and is more 

 readily decomposable than the denser lava. 



It seems evident that the diamond-bearing pipes 

 are true volcanic necks, composed of a very basic 

 lava associated with a volcanic breccia and with tuff, 

 and that the diamonds are secondary minerals pro- 

 duced by the reaction of this lava, with heat and 

 pressure, on the carbonaceous shales in contact with 

 and enveloped by it. 



The researches of Zirkel, Bonney, Judd, and others, 

 have brought to light many eruptive peridotites, and 



