198 J. C. BRANNER AGGRADED LIMESTONE PLAINS^ BAHIA^ BRAZIL 



a mountainous region the jDrecipitation must have been proportionately 

 greater in this part of Brazil than it would be in temperate regions. 



The period of elevation and greater precipitation was followed by a 

 period of depression, and consequently of diminished rainfall. The sec- 

 ondary streams were so enfeebled that they were no longer able to keep 

 their channels clear, and these began to be choked up with lime. In those 

 instances where the streams head on low gentle slopes the channels were 

 quickly filled, and they have even been encroached on by the surrounding 

 forests. With the larger secondary streams, such as the Eio Salitre, the 

 process is still in operation, but it has already gone so far that the vegeta- 

 tion is now encroaching on the almost completely abandoned water- 

 courses. 



Briefly, then, the explanation of the extensive chemical aggradation 

 going on over the limestone regions of the interior of Bahia appears to be 

 due to a diminished rainfall in a semi-arid region, while the diminished 

 rainfall appears to be due to the depression of the northeastern part of 

 the continent since Miocene times. 



An older Catinga Limestone 



Within the immediate basin of the Eio Sao Francisco there are exten- 

 sive deposits of limestone that differ so widely in character and distribu- 

 tion from the Catinga limestones of the Salitre Valley that it looks at 

 first glance as if the explanation oftered for the Salitre deposits were not 

 applicable to the deposits along the margins of the Sao Francisco flood- 

 plain. Inasmuch as those older deposits have been formed under some- 

 what different circumstances, it is necessary to describe them in a little 

 more detail. 



The most accessible and best known of the older Catinga limestones is 

 a series exposed along the line of the Sao Francisco Eailway, between the 

 stations of Angico and Piranga. This rock, however, is very hard and 

 compact, so much so that it is much used for buildings and pavements. 

 It was formerly supposed that this limestone was Paleozoic,^ ^ but later it 

 was judged to be a fresh-water deposit "of Tertiary or possibly Quater- 

 nary age," ^^ though no fossils had been found in the rocks at that time. 



One of the most striking characteristics of this particular Catinga lime- 

 stone is its geographic and hypsometric distribution, and its hardness and 

 compactness as compared with the Catinga limestones of the upper 

 Salitre Valley. The distribution is fairly evident along the line of the 



11 Theodoro F. Sampaio : Revista de Engenharia, 14 de Margo de 1884, pp. 52-54. 



12 O. A. Derby : Jour. Geol., vol. xiv, Chicago, 1906, p. 380. 



