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



Page 



Eainfall 309 



Rate of d.ecom])Osition 312 



Resume 313 



Introduction. 



In writing of Brazil, Louis Agassiz^ says that — 



''The decomposition of the surface rocks to the extent to which it takes place 

 is very remarkable, and points to a new geological agency thus far not discussed in 

 our geological theories. It is obvious here . . . that the warm rains falling 

 upon the heated soil must have a very powerful action in accelerating the decom- 

 position of rocks. It is like torrents of hot water falling for ages in succession upon 

 hot stones. Think of the effect, and instead of wondering at the large amount of 

 decomposed rocks which you meet everywhere you will be surprised that there are 

 any rocks left in their primitive condition." * 



B}'' decomposition, decay and disintegration as used in the present 

 paper I refer only to the phase or i:)hases of rock decay which can be 

 detected by the eye. I shall not attempt any detailed analysis of the 

 chemical processes of decomposition or of the mineral changes. These 

 changes I assume to be the same in the main as those accompanying 

 rock decomposition in other parts of the world, with differences only in 

 the rate at which it goes on and in the degree of oxidation known to exist 

 between warm and cold climates.f No distinction is made between dis- 

 integration and decomposition, for the former is believed to be incipient 

 decay, however slight the chemical changes may be. 



Evidences and Results of Decomposition. 

 decay ix place. 



General distribution and character of the decomposition. — Disintegration of 

 rocks in Brazil is both profound and widespread. The w^orking out of 

 structural geolog}^ over limited areas is often made altogether imj^ossible 

 by the breaking down of the stratification and by the mingling of the 

 products of decomposition in land-slides and by the creep of the soil, 

 while the decay of the crystalline rocks often renders the determination 

 of their constituent minerals difficult or altogether impossible. 



This deep decomposition is not confined to any particular part of the 

 country, but it is a pretty constant feature of the geology from the equator 



* Journal in Brazil, 89. Am. Jour. Sci!, 2d ser., vol. xl, 1865, p. 390. 



fSee Subaerial decay of rocks and the origin of the red color of certain formations. Bull. 52 

 U. S. Geol. Survey. I. C. Russell. This bulletin contains a partial bibliography of the subject of 

 rock decay. 



