144 OR VILLE A . DERB Y 



tory than those to the other two. At the time of Burton's 

 visit the most typical pegmatitic body was regarded as the 

 richest of the Duro mine and in his description of the Barro 

 mine he states that the white clay (called diz, or chalk, by the 

 miners) served as a guide to the diamond formation. It is by 

 no means certain, however, that the diamonds actually occurred 

 in it rather than in the adjacent colored clays in contact with it 

 and for which it serves as the most apparent guide. In the 

 specimens at hand the part considered as the contact zone is 

 mineralogically the richest, and it may be suspected that the 

 diamonds occur in it rather than in the white clay. The lower 

 body reputed to be the richest at the time of Gorceix's visit is, 

 according to his description and that of Burton, much less 

 decidedly pegmatitic in aspect and the part seen by me seemed 

 to be a decomposed dike with no apparent suggestion of peg- 

 matite. The part of this body indicated to me as diamantif- 

 erous belongs certainly to the supposed dike and not to the 

 contact zone. The other body, the Barro Preto, seems, accord- 

 ing to the descriptions and the part seen by me, to be a special- 

 ized layer of the phyllites the relations of which to the peg- 

 matites (if it has any) are not clear. In short the question as 

 to whether the diamond occurs at Sao Joao da Chapada exclu- 

 sively in the supposed pegmatitic bodies, in the contact zone of 

 said bodies, in layers of the phyllites more or less removed 

 from such contact zones, or in all, must remain an open one. 



As the case stands at present the indications seem to be 

 rather in favor of the hypothesis of the formation of the dia- 

 mond in the phyllites, the presumptive agent being the sup- 

 posed eruptive which in some of its phases presents a pegmatitic 

 character. This involves, presumably though perhaps not neces- 

 sarily, the supply of the necessary carbon from the phyllites 

 themselves, but as the series is known to include in many places 

 graphitic members such a supply may reasonably be predicted 

 at Sao Joao da Chapada. Moreover the evidence from Kim- 

 berley, where, according to Launay [Les Diamants du Cap), the 

 rock considered rich only yields one part of diamond to 3 mil- 



