SPECULATIONS REGARDING THE GENESIS OF THE 



DIAMOND 



ORVILLE A. DERBY 

 Rio de Janeiro 



The recent admirable summary by Dr. Percy A. Wagner 1 of 

 what is now definitely known regarding the geological conditions 

 in which the diamond occurs in South Africa suggests certain specu- 

 lative points of view, which, if found worthy of attention, may in 

 turn suggest desirable lines of investigation in the field and the labo- 

 ratory. These inquiries may perchance throw light on the intricate, 

 fascinating question of the genesis of the diamond; or, even in 

 a broader way, on the role of carbon in eruptive rocks, whether in 

 the form of diamond or graphite, or as gas locked up in carbonates 

 or certain silicates. 



To the student of the occurrence of the diamond in countries 

 other than South Africa, one of the most significant facts established 

 by the prospecting of the African miners is that, aside from its 

 well-known occurrence in pipes, the diamond-bearing eruptive 

 material, kimberlite, occurs also in dikes, and that these usually 

 have considerable longitudinal extension but only small width, 

 except where expanded into pipes, and even these are frequently 

 insignificant in relative dimensions. This slight prominence of the 

 diamond-bearing bodies, coupled with the extreme susceptibility 

 of the material to alterations which render its identification a matter 

 of great difficulty, suggests at once that the failure to detect such 

 dikes and pipes in districts in which the diamond is found only in 

 sedimentary deposits, modern or ancient, is not a conclusive argu- 

 ment against their existence, nor is it clear evidence that the original 

 matrix was notably different from the South African kimberlite. 

 In countries like India and Brazil, in which the diamonds of the 

 modern alluvial deposits have been definitely traced back to con- 

 glomerates of considerable geological age, the presence or absence 

 of kimberlite dikes should be tested by prospecting operations, 



1 Die diamantfiihrenden Gesleine Siidafrikas, Berlin, 1909. 



627 



