ON THE GENESIS OF THE DIAMOND 1 45 



Jon to 36 million parts of rock, indicates that the amount required 

 is so infinitely small that few rocks can be conceived that may 

 not contain, in some form, the necessary supply of carbon. The 

 amount of this element that presents itself in the form of car- 

 bonates in the decay of many rocks, that in their sound condition 

 are not recognized as containing it in any form, is far in excess 

 of that here indicated as necessary. In this connection it may 

 be remarked that the hypothesis that attributes a preponderant 

 importance in the genesis of the diamond to the carbonaceous 

 shales of the upper part of the Kimberley section, is subject to 

 the criticism of furnishing a preposterously enormous super- 

 abundance of raw material. 



The three localities above discussed offer no certain indica- 

 tions of more than one mode of genesis of the diamond in 

 Brazil. The occurrences at Grao Mogol and San Joao da Cha- 

 pada can very easily be brought into line on the hypothesis, 

 which has much in its favor that at the former place the diamond 

 is an allothigenic mineral derived from deposits similar to those 

 at the latter. For Sao Joao da Chapada and Agua Suja the 

 comparison presents no difficulty if the diamonds at the last 

 place are assumed to come, as is quite possible, from the under- 

 lying schists. If, however, they are genetically related to the 

 later eruptive series, the hypothesis of a substantially similar 

 mode of genesis requires that the predominant factor be an 

 eruptive rock, which may vary greatly in its mineralogical char- 

 acter and mode of occurrence. 



As compared with the Kimberley occurrence that of Sao 

 Joao da Chapada seems at first sight to be characterized by an 

 almost absolute lack of analogies. Until quite recently the only 

 known feature at Kimberley offering some remote resemblance 

 to the Brazilian fields was the presence of a quartzite in the 

 lower part of the section. This resemblance is somewhat 

 increased by the later developments as metamorphic schists 

 appear mingled with the quartzite in the lower levels of the 

 deep shaft (see section on p. 137 of Launay's Les Diamants du 

 Cap). For the present the information regarding these lower 



