192 J . C. BKANKER AGGRADED LIMESTONE PLAINS, BAHIA, BRAZIL 



tains that extend from the Serra da Crnz on the north to the Serra de Sao 

 Mauricio, about 100 kilometers north of Morro do Chapeo. This western 

 rim is made up of the Lavras beds and other old sediments that dip be- 

 neath the valley. Within this basin the Salitre limestones overlie the 

 older formations, filling embayments, lapping well np over its eastern 

 margin, and encircling the hills that rose as islands from the waters in 

 which they were deposited. The Salitre limestone was the last great 

 series of rocks to be deposited in the Salitre Valley. For this reason lime- 

 stones are the surface rocks over most of the valley floor. The only excep- 

 tion worthy of mention here is a series of gravels and sands that flank the 

 eastern base of the mountains along the west side of the valley. There 

 should also be excepted the Catinga limestones, which are widely dis- 

 tributed over certain portions of the valley. 



Origin op the Catinga^ Limestones 



To the character, distribution, origin, and modification of the Catinga 

 limestones in the Salitre Valley especial attention is directed. As sug- 

 gested in the table of formations in this region, the Catinga limestones 

 are of recent origin. It is evident, also, that they are derived either 

 directly or indirectly from the much older Salitre limestones or from any 

 other limestone that happens to be at hand. Wherever the Catinga lime- 

 stone covers any considerable area, the beds are horizontal or approxi- 

 mately so. 



At many places temporary shallow lakes are formed during the wet 

 season. The limy soils of the Salitre A^^alley are everywhere favorable for 

 land and fresh water mollusks, and large numbers of the shells of the 

 animals often accumulate in and about these temporary pools. On the 

 fazenda Varzea do Sal I found the weeds growing over one of these shal- 

 low lakes literally covered with the animals that had crawled up their 

 stems, apparently to hibernate during the dry season, while over a wide 

 adjoining area the shells had been cemented into a compact limestone. 

 (See plate 15, figure 1.) Such instances are common through all the 

 flat limestone region. 



Old stream beds are filled up with these recent deposits ; for long dis- 

 tances the stream channel of the Eio Salitre has been filled until it is as 

 flat as a floor. This is especially true of the part of the Salitre Valley 

 above Pacuhy, or, to speak more comprehensively, it is true of those por- 

 tions of the valley in which the streams are not perennial. 



' "Catinga" is the Brazilian name of the short, scrubby forests that are chai-acteristic 

 of most of the semi-arid regions of the interior. 



