BLACK DIAifOND REGION OP BRAZIL. 



325 



upon the geologic history of the region. The following table shows 

 the sequence and the chief subdivisions of the rocks of the region in 

 so far as they are now known : — 



In Mr. Derby's paper on the Bahia diamond region he. speaks of 

 the Lavras series and of an underlying series which he calls the 

 Paraguassu.;]: The later work of the writer shows that the diamonds 

 and carbonados are not confined to any one horizon, and that, 

 therefore, the Paraguassu series of Derby can only be regarded as a 

 local subdivision of the entire series. Mr. Derby's name, Lavras, seems 

 very suitable for the whole diamond-bearing series, however; and it 

 is, tiierefore, retained for the entire series. 



It should be distinctly understood that, while the divisions here 

 suggested are perfectly clear, the ages assigned all of those below the 

 Cretaceous are without pala^ontological warrant, and must therefore 

 stand subject to such readjuj^tment.s as future discoveries may require. 

 The stratigrapliic relations, however, are known to be correct, but 

 tJK- thicknesses vaiy as usual from one place to another, and in some 

 places some of the members are altogether wanting. 



It Avill be noted that the Cambao quartzites and the Jacuipe flints 

 are not assigned to any age. This is because it is not clear whether 

 these divisions belong with those above or below — that is, whether 

 the Jacuipe flints are Silurian or Devonian. 



TriE Diamond Bearing Beds. — The diamonds and carbonados are 

 found in the Lavras series of this section. The rocks of the Lavras 

 series are pinkish quartzitic sandstones and conglomerates with some 

 interbedded clays. The coarser beds are strongly false-bedded, and the 

 series is faulted in some places, and is everywhere folded and denuded. 

 The rocks of the series are thus bunched as synclines in some places, 

 and in others they are separated by denudation into isolated patches. 



There are no eruptives in the diamond-bearing beds as a whole, 

 but there are some basic dykes cutting them at a few places. These 

 dykes are diabase-like rocks, but, inasmuch as the diamonds are 

 found more than 100 miles from any known dykes of this kind, these 

 particular eruptives clearly have no genetic relations to the diamonds. 



t Economic Geology. I., 134-142, Dec. 1905. 



