THE 



JOURNAL OF GEOLOGY 



FEBRUARY-MARCH, 1898 



BRAZILIAN EVIDENCE ON THE GENESIS OF THE 



DIAMOND. 



The extensive working of the " dry diggings " of South Africa 

 has thrown a light on the original associates and probable mode 

 of origin of the diamond, which it would be vain to look for in 

 the ordinary type of diamond fields as known in other parts of 

 the world, since all of these, with rare and imperfectly known 

 exceptions, correspond almost exactly with the river washings 

 of the Cape district. The group of mines about Kimberley have 

 shown beyond a doubt that here at least the association of the 

 diamond is with an eruptive rock of ultra basic composition,, 

 and, although opinions differ as to the exact mode of origin, all 

 authorities seem to agree on the main fact that in some way this, 

 association is a genetic one. To one familiar with the Brazilian 

 diamond fields this conclusion seems a startling one and utterly 

 inapplicable to them. The evidence in its favor, if any exists, 

 is either concealed and has been overlooked, or, as at first sight 

 seems most probable, a totally different association is presented, 

 necessitating the hypothesis of the formation of the mineral 

 under a far wider range of conditions than has been admitted 

 by the students of the Kimberley occurrence. In the present 

 paper it is proposed to discuss the observations, in great part 

 unpublished, that have thus far been made in Brazil bearing upon 

 the question of the genesis of the diamond, with a view of see- 

 ing in how far they are in accord, or disaccord, with the much 

 more complete observations in the South African mines. 

 Vol. VI, No. 2. 121 



