GENERAL AND ECONOMIC GEOLOGY 



235 



Joazeiro to the southern end of the State of Bahia, and extending into 

 Minas beyond, is a mountainous and hilly region averaging 200 kilo- 

 meters in width, mostly of Paleozoic sediments folded, faulted, and de- 

 nuded. They are made up chiefly of hard quartzitic sandstones, shales, 

 and slates covered here and there by limestones. The geologic ages of 



Figure Q.—East-ivest Section across the Serra do Tomhador, the Serra de JacoUna, and 

 the Archean Valley hettceen 



these rocks are not certainly known; for, with the exception of certain 

 limestones to be mentioned later, not one of them has yet yielded recog- 

 nizable fossils. 



The lowest rocks of this group, the Tombador beds, have been referred 

 tentatively to the Silurian. These lowest of the sedimentary rocks form 

 the escarpment of the Tombador range west of Jacobina, the cap rock of 



. dLa, Crua 



SerroL, JLoTTulotb 



Figure 7.— Structural Relations of Serra da Cruz, Serra do Boqueirao, and Serra do 



Mulato 

 The crests are of Tombador quartzite ; the slopes and plains are of old crystalline rocks. 



the Serra do Mulato, Serra da Cruz, Serra do Encaibro, Serra de Macau- 

 bas, and Serra Geral on Eio Sao Francisco, and. they generally underlie 

 the later rocks of the diamond mining regions of the State of Bahia. 



The limestones at Bom Jesus da Lapa on Eio Sao Francisco have been 

 referred by Derby to the Silurian on the evidence of the fossils Favosites 



{^\J ~ Serra do Tom bado 

 RioSFco. 



Serra Pedro Branca. Cascavel 



. Alrrias Valle\ 



Figure S.—Xorth^cest-southeast Section from Rio Sao Francisco to the Salitre Yalley, 



alout 160 Kilometers 

 The section shows the general geologic structure of the diamond region of Bahia. The 

 Caboclo shales are exposed in the Almas Valley, with the diamond-bearing beds overlying 

 them. 



and Chetetes found there. This determination, however, is not to be 

 regarded as final, for both of these fossils occur also in the Devonian, and 

 the Chetetes is even found in the Carboniferous. 



Next above the Tombador beds is a series of slaty shales well exposed 

 in the Almas Valley, and called the "Caboclo shales" for the Caboclo peak 



i 



