113 



phosed to graphite. But hDw it was enclosed in igneous rocks as 

 a secondary product of a previously existing organic accumulation 

 is a question that has still to be answered. It is suggested, how- 

 ever, that petroleum, whether the product of organic substance or 

 of some chemical interaction upon metallic carbides, made its 

 way along veins and there deposited carbon, which in the lapse 

 of time became graphite. One must confess that it is difficult 

 to avoid the conclusion that some at least, if not the greater part, 

 of the graphite found in rocks was derived from a purely inorganic 

 source. 



The diamond is the purest form of carbon found in nature. 

 The geological features of its occurrence are matters of great 

 scientific interest. In the greater number of its sources it occurs 

 as isolated crystals in river wastes, in gravels, sands and con- 

 glomerates, that is in beds quite evidently derived from pre- 

 existing rocks elsewhere. The diamonds of India, Brazil, Borneo 

 and some localities in South Africa are alluvial in origin, and no- 

 where is the parent crystalline rock in which the diamond was con- 

 ceivably generated to be seen. The Kimberley diamond fields are, 

 however, of a different type. Here the gems occur as etched 

 octahedra with curved edges and a greasy lustre, enclosed in a 

 matrix, known as " blue ground." This blue ground is a green 

 serpentinous breccia, which contains grains of an extraordinary 

 variety of minerals ; among them, garnets, green pyroxenes, altered 

 magnesian mica and black ilmenite. It is evidently a much rotted 

 volcanic rock. This volcanic agglomerate is confined to vertical 

 pipes of unknown depth, some 200 yards or more across at the 

 surface. The diamonds are scattered through the blue ground, 

 generally loosely, but rarely as enclosures in fragments of a 

 schistose rock, eclogite. 



The question, how did the diamonds get into the pipes has 

 called forth many suggestions. One suggestion is hazarded that 

 the diamonds were encased in the body of the meteorites that 

 rained down at some distant period in a rapid shower and have 

 since been released by the decay of the meteoritic minerals. The 

 so-called pipes, on this theory, would be simply holes drilled into 

 the earth's crust by the tremendous impact of the meteorites. 

 A remarkable feature, the Arizona meteoritic plain, supplies some 

 sort of confirmatory evidence for this idea. There a broad plain 

 five miles in width is scattered over by some 2,000 masses of 

 metallic iron plainly of meteoritic origin. Further, on the plain 

 rises a crater witfi" raised unbroken edges, three-quarters of a mile 

 across and 600 feet deep. The only possible explanation appears 

 to demand some powerful impact of a colossal meteorite, remnants 

 of which lie scattered on the plain outside. Some of these frag- 

 ments actually contain black and transparent diamonds, besides 

 crystals of carbon silicide. 



Another more probable supposition is that the pipes consist 

 essentially of an -altered peridotite, an olivine rock from which 

 the diamond has crystallised. Friedlander following on the mode 



