CLIMATE OF THE REGION 195 



caverns. The face of the bluff is covered by stalactitic projections which unite 

 to form a good many of the caves. . . . This is the same limestone as that 

 covering the plains south of Joazeiro." 



The process as a whole is one of chemical aggradation; of leveling 

 down of the older limestones by solution on the higher ground, and of 

 leveling up by deposition from solution at somewhat, lower elevations. It 

 is to be noted, however, that the distance moved is not necessarily great ; 

 that the water is shallow and not concentrated in well defined streams; 

 that the process is a slow creeping one, carried on over wide areas and for 

 long periods. 



The Climate 



The climatic conditions of the region are of the utmost importance in 

 connection with the processes of chemical aggradation. The rainfall over 

 this catinga-covered country is confined to a few months in the year, and 

 it is never great. The first waters soak quickly into the ground, for the 

 surface is very dry after months of tropical sunshine and the hot winds 

 that blow for months of the jear over this region. When, at times of 

 rain, there is more than enough water to dampen the surface of the 

 ground, it starts to flow, but it does not go far before it all soaks into the 

 ground. When the forest litter has once been thoroughly wet, decompo- 

 sition is rapid, and the carbon dioxide formed helps attack the lime and 

 to move it forward in the direction of the drainage. The dryness of the 

 earth to considerable depths, however, does not permit the waters to flow 

 away as freely as they would in regions of more evenly distributed rain- 

 fall. Where the waters are shallow the sun warms them, drives off the 

 carbon dioxide, and the lime is precipitated, after being carried only a 

 short distance. This precipitation may take place either in a shallow 

 stream, in a shallow lake or pool, where the water moves in a sheet over a 

 flat surface, or where its grade is abruptly changed at the edge of a bluff 

 or slope of the land. Wherever the waters are completely evaporated all 

 the lime is finally precipitated along with other minerals in solution. 

 This process of aggradation has been in operation so long that the Salitre 

 Valley has been affected by it wherever the limestones are the surface 

 rocks. The rocks of the higher grounds are removed in solution, and 

 little by little the lower grounds are built up. The less soluble parts of 

 the limestones are commonly left behind, so that the high ground is often 

 covered with irregular lumps and masses of flint, chalcedony, or quartz 

 sand. Limestone hills are usually capped with quantities of such frag- 

 ments, which sometimes lead one to imagine that there must be a series 

 of siliceous rocks overlying the limestones. Thus far no such series has 



