Carbonados in the State of Bahia, Brazil. 489 



garded as helpful in determining the Brazilian association of 

 diamonds. 



Omitting from the list all minerals of common or wide occur- 

 rence, I have made up the following table of those which I 

 regard as diagnostic in the state of Bahia, and the columns indi- 

 cate their genetic rock associations : 



Table of Diagnostic Minerals associated ivith Diamonds 

 and Carbondos in Bahia. 



Deep- 

 Erup- seated Metamor- Pegma- 



tives granites phics tites 



Beryl _ . .. .. x x 



Brookite x x 



Cassiterite .. . - . - x 



Columbite -. .. ... x 



Corundum .. x x 



Cyanite . . . . x 



Diaspore . . .. x 



Monazite .. x x 



Octahedrite . .. .. x 



Staurolite . . . . x? x 



Tantalite ._ .. .. x 



Titanite ..'. x x 



Xenotime .. x .. x 



Zircon x x 



This table brings out the fact that the genetic relations of 

 these minerals are plainly with deep-seated granites, metamor- 

 phics, and pegmatites rather than with eruptives. 



In view of the widespread occurrence of diamonds in the 

 Lavras quartzites in Bahia, and in view of the absence of 

 eruptives from the vicinity of the diamond washings, one of 

 the following conclusions seems to be warranted : 



First, either the Bahia diamonds or carbonados originated in 

 the old granites and metamorphics that underlie the Paleozoic 

 beds in Bahia and have passed down mechanically to their 

 present position, or 



Second, they are one of the results of metamorphism of the 

 quartzites themselves and have originated where we now find 

 them. 



Possible origin of the diagnostic minerals. — If the diamonds 

 and their associated diagnostic minerals were derived from 

 older metamorphic or eruptive rocks, where did they come 

 from ? East, west, and south of the diamond regions of Bahia 

 are extensive areas of the crystalline rocks mentioned at the 

 beginning of this article, consisting of old, probably Archean, 

 granites, gneisses, schists, and eruptives of many kinds, and 

 similar to those upon which subsequent deposits have been laid 

 down. If, therefore, such a source is assumed for the diamonds 



