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



10. An important factor in the decomposition of the rocks is the fact 

 that the rainy season is the hot season, and the waters falling upon hot 

 rocks have their temperatures raised to about 140 degrees. 



11. The dark color of the rocks increases their al)sorl)ing and radiating 

 power and consequently the range of temperatures to which they are 

 subject. 



12. The unequal expansion and contraction of minerals with changing 

 temperatures hasten the disintegration of rock surfaces. 



13. The mechanical effects of changes of temperature, however, are 

 surface phenomena. 



14. The coarse textured rocks seem to be more susceptible to decom- 

 posing agencies than the more compact ones. 



15. Insects living in the ground, especially the ants and termites, con- 

 tribute large quantities of organic acids to rock decomposition. 



16. Plant life is especially rank, and both growth and decay are more 

 rapid than they are in temperate regions. Plants are the chief source of 

 the acids which attack the rocks of Brazil. 



17. Carbonic acid is also brought to the earth in large quantities by 

 the rains. 



18. Nitric acid is produced and brought down by the rains in much 

 larger quantities than in temperate regions. 



19. The rainfall of Brazil is very large, ranging from 974.6 millimeters 

 at Rio de Janeiro to 3,576 millimeters on the Serra do Mar, in tlie state 

 of Sao Paulo. 



20. The concentration of the rainfall renders it more effective both 

 chemically and mechanically. 



