ON THE GENESIS OF THE DIAMOND 



131 



ments of this rock. If it belongs to the former class, two modes 

 of genesis must be admitted in the same field in order to recon- 

 cile the occurrence with that at Sao Joao da Chapada ; if to the 

 latter, the two occurrences can be explained on the hypothesis 

 of a single mode of genesis, but, in case the series proves to be 

 a single one, two periods of formation must be admitted. 



For the question of genesis the most significant of the Bra- 

 zilian localities is that of Sao Joao da Chapada near Diamantina 



Fig. 2. — Diamond mine at Sao Joao de Chapada. The view is looking north- 

 ward into the Barro mine, which runs into the Duro under the footbridge. The 

 diamond-bearing layers dip into the bank at the right at an angle of about 50 . The 

 open space in the foreground where the wash-house, concentration tanks (canoas), and 

 heaps of diamond-bearing earth are located, represent the excavated portion of the 

 left bank containing the upward prolongation of the diamantiferous layers. The 

 wide trench near the pump-house at the left is apparently on the outcrop of one of 

 the layers. 



which has never been satisfactorily studied and described. The 

 accompanying view, drawn from a photograph, which is also 

 reproduced in Boutan's monograph on the Diamond in Fremy's 

 Enclyclopedie Chimiqtie, shows its character as a great gash resem- 

 bling a railroad cutting across the crest of a high ridge that forms 

 part of the divide between the waters of the Jequitinhonha and 



