AN OLDER CATINGA LlMESTOlSTE 203 



Following this Catinga limestone to the southwest, the only thing that 

 seems worthy of especial note is that it is cut out by all streams; that 

 above certain levels it disappears where it enwraps the peaks of quartzite 

 and other older rocks that rise above the plain, and that in some places 

 water-worn boulders of quartz and quartzite are found imbedded in it that 

 measure as much as 12 by 8 by 4 centimeters, and with these are many 

 smaller boulders. No fossils were found in these exposures, but the rock 

 looks promising for fossils. 



Inquiry regarding the limestone at or about this same elevation over 

 the river plain of the Sao Francisco brings out the fact that the horizon- 

 tality of the deposit, its thickness, its position on the low watersheds, and 

 its absence from the immediate stream channels are pretty constant char- 

 acteristics over the ancient floodplain of this section of the river. 



It is reported to occur over the low land along the Eio Sao Francisco 

 as far up as Bom Jesus da Lapa, and along the Eio Corrente and Carin- 

 hanha, on the west side of the Sao Francisco. Below Joazeiro it is re- 

 ported as far east as the valley of the Eio Patamute. 



In the region about Joazeiro the Catinga limestone, so far as now 

 known, is confined to an elevation below 500 meters above tide, and most 

 of it is above 400 meters. Data are not at hand to show its elevation 

 higher up the Sao Francisco, but in view of the conditions under which 

 it was deposited it is expected that it will maintain a rather even vertical 

 range between 40 and 125 meters above the ordinary stages of that stream. 



Origin of the Catinga Limestone op the Sao Francisco Flood- 

 plain 



In connection with the theory of the origin of this older Catinga lime- 

 stone it should be mentioned that when the question of its origin first 

 presented itself the theory of its having formed in a shallow lake was 

 entertained. In the presence of the facts gathered this theory does not 

 seem to be tenable ; nor does the explanation of the origin of the Catinga 

 limestone in such areas as the Salitre, the Jacare, and the Eio Verde 

 valleys seem to explain the extensive deposits on the high margins of the 

 immediate valley of the Eio Sao Francisco. The process, however, is be- 

 lieved to be only a modification of the one now in operation in the lateral 

 valleys. The distribution of these older beds show that they were de- 

 posited in the shallow waters of the Sao Francisco itself when those 

 waters spread over the area in which these rocks are now found. I believe 

 they belong to the period of heavy Miocene rainfall, when the region 

 stood considerably higher than it does at present. During that period 

 the floods of the Eio Sao Francisco were greater than they are now, and 

 XV — Bull. Geol. Soc. Am., Vol. 22, 1910 



