Diamond Mines: Diamond Formation, Formation of the Diamond Page 162



	132 THE DIAMOND MINES OF SOUTH AFRICA bonaceous shales into cavities of the amygdaloidal rock which lies outside of the volcanic pipes. Then he reduces this gas by enormous pressure to a liquid state, and, having gotten it into a form, as he thought, admitting of crystallization, he absorbs the oxygen of the carbonic acid by the iron in the containing walls of the craters. Now, as a matter of fact, there are no cavities in the amygdaloidal rock underlying the shales, for all interstices are filled with silica in the form of agates, or with calcite. Furthermore, if carbonic acid had been left in the olivine diabase to crystallize, then the resultant diamonds would have been enclosed in this formation, which is also contrary to fact, for no diamonds have ever been found in the amygdaloidal rock. His main contention, too, is the derival from the shales of the carbon necessary for the formation of diamonds. It will be made clear, subsequently in this discussion, that this assumption is not justified. The late Henry Carvill Lewis, M.A., F.G.S., Professor of Mineralogy in the Academy of Natural Sciences, Philadelphia, U.S.A., advanced the proposition that the diamond is the result of the intrusion of igneous rocks into and through the carbonaceous shales, and the crystallization of the carbon throughout the rocks, as it cools, from hydrocarbon, distilled from the shales that had been broken through.¹ In support of such a theory, it is claimed that the diamonds in the various mines or pipes have different characteristics. It is quite true that large parcels of diamonds from the various mines have distinctive characteristics, and it can be easily told from which mine a parcel of diamonds comes ; but it is very difficult to tell in which mine a single stone may have been found, though each mine has stones in a great measure peculiar to itself. Some observers claim that the broken diamonds which are extracted are broken during the process of winning them. It is admitted that diamonds may be broken in the process of mining and the subsequent operations of winning, but these cases are exceptional. Fragments of diamonds are very frequently found embedded in the blue ground, and there ¹ "The Matrix of the Diamond," Professor Henry Carvill Lewis.