ON THE GENESIS OF THE DIAMOND 



129 



■of the unity of the formation, since elsewhere the diamond was 

 considered to be authigenic under different conditions in a 

 different rock referred to the same geological series. 



The question of the unity of the section at Grao Mogol and 

 of the clastic origin of at least the upper portion to which, 

 according to all authorities, the diamond is confined, is an 

 important one in this connection, since, if Gorceix's view is cor- 

 rect, it involves that of two widely different modes of genesis 

 for the diamond in the same field. Not having seen the place, 

 it is with considerable diffidence that I venture to contest the 

 views of the eminent authorities who have. 



The accompanying figure, reproduced from one of Helm- 

 reichen's sketches, gives what seems to me to be a more accurate 



Fig. 1. — Left bank of Corrego dos Bois, near Grao Mogol showing diamond- 

 bearing bed (a, b). After Helmreichen. 



representation of the actual aspect of the exposure than the 

 somewhat diagrammatic section given by Gorceix, and it has the 

 further element of authenticity of representing, at the base of 

 the diamond-bearing bed [a, $),an apparent unconformability 

 that is not in accord with the theoretic views of the author of 

 the sketch. Numerous cases of a break in the succession have, 

 on close examination, been observed in the Diamantina district 

 that are much less apparent in the topographical features than 



