312 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



Maximum and Minimum Rainfall by Trimesters. 



Stations. 



Rio de Janeiro .... 



Santos 



Alto da Serra 



Silo Paulo 



Alto Parnaliyba * . 



Uberaba 



Morro Velho 



Gonjjo Soco 



Itabira* , 



Queluz 



INIanuus, Amazonas 



Para 



Cearu 



Pernambuco 



Victoria 



Colonia Isabel .... 

 S. Bento das Lages 

 Baliia . 



INIaximum rain- 



Minimum rain- 



fall for three 



fall for three 



m o n t h s (in 



m o n t h s (in 



millimeters). 



millimeters). 



349.3 



135.9 



1,000.2 



3()0.7 



1,308.0 



587.5 



810.7 



111.9 



563.2 



0.0 



840.9 



68.0 



923.0 



39.0 



1,715.5 



109.4 



732.9 



0.0 



943.9 



53.8 



955.4 



176.8 



871.6 



128.7 



941.8 



41.4 



1,682.5 



106.7 



469.2 



61.6 



492.4 



64.6 



981.5 



235.4 



1,156.3 



256.7 



The little rain that falls during the dn^ months is not enough to fill 

 the streams, but it all or nearly all soaks into the dry ground at a time 

 when it is highly effective as a chemical agent. The effect of long dry 

 periods upon the soil should not be overlooked. In many places, espe- 

 cially in the claj^ey lands and in the soils derived from the calcareous 

 rocks of the Cretaceous l)elt, great cracks open the soil to a dei)th of from 

 5 to 10 feet, according to tlie length of the dry season. These crevices 

 admit atmospheric air and gases readily to a considerable depth, organic 

 matter is constantly falling into them, and when rains come the surface 

 waters penetrate at once to their bottoms and fill the whole upper soil. 



It is worthy of note that so far as our defective statistics go they show 

 that the rainfall is largest along the east coast of l^razil, where the south- 

 east air currents from the Atlantic first strike the continent. Tliis region 

 also includes the principal gneiss and granite area of Brazil and the re- 

 gion of decomposed schists and shales of the Minas water-shed. 



Rate of Decomposition. 



No data are at hand for an exact determination of the rate of rock de- 

 composition in Brazil. Some of the oldest gneiss buildings do not 

 exhibit any marked evidences of decay, while others are clearly soft- 



'■^- Observations for one year only. 



