ON THE GENESIS OF THE DIAMOND 1 43 



— have not as yet been definitely identified in Brazil, though 

 they doubtless occur. The apparent absence (or extreme rarity) 

 of zircon may perhaps be taken as indicative of gabbro, and con- 

 siderable masses of this type of rock, to which the supposed 

 pegmatite might be referred as apophyses, are known to occur 

 in the diamond region. So far as known, however, this is the 

 utmost limit in the direction of basic rock types to which one 

 can go, even hypothetically, in seeking the probable original 

 type of this supposed pegmatite. 



The question of the eruptive or secretionary origin of peg- 

 matites has long been a subject of discussion among geologists, 

 and eminent authorities can be cited in favor of either view. 

 The recent studies of Lehmann, Brogger, Williams, Crosby, and 

 others seem to have clearly established that most if not all of 

 them are essentially eruptive masses, though possibly modified 

 in some way by aqueous agencies. Even before becoming 

 acquainted with the literature of the subject this view had 

 seemed to me to be the only acceptable one as regards the typ- 

 ical pegmatites of Brazil. The extension of it to such prob- 

 lematic occurrences as the diamond-bearing bodies of Sao Joao 

 da Chapada and the topaz-bearing bodies of Ouro Preto cannot 

 as yet be fully established on account of the lack of complete 

 studies in the field and the decomposed condition of the mate- 

 rial. Aside from the general analogy that they present with 

 typical pegmatites, nearly all the criteria given in the recent 

 papers by Williams and Crosby and Fuller in support of the 

 hypothesis of eruptive origin can be cited in favor of the same 

 hypothesis as applied to these bodies. If, as is suspected, they 

 present phenomena of contact metamorphism, a crucial test 

 can be applied through the study of the heavy residues of the 

 enclosing schists at different distances from the contact. This, 

 however, involves field studies that for the present cannot be 

 undertaken. As the case stands at present the hypothesis of 

 an eruptive origin, though not fully proven, is by far the most 

 probable. 



The response to the third question is still more unsatisfac- 



