DECOMPOSnVO.V OF ROCKS IN BRAZIL 537 



contrasted may l^c taken as approximate!}' equal. So far as the- 

 present evidence goes, however, it is to the effect that in the dis- 

 tricts of highly inclined crystalline and metamorphic schists of the 

 states of Minas Geraes and Rio de Janeiro profound rock decom- 

 position is not a concomitant of the present distribution of forests. 



An interesting feature in the decay of some types of granite 

 is that a considerable portion of the feldspar resists decomposi- 

 tion almost as well as the quartz and the rock disintegrates into 

 a coarse gravel with comparatively little earthy, or completely 

 decomposed, elements. This was first noticed on any consid- 

 erable scale in a relatively arid region along the Sao Francisco 

 River and was thought to be a case of arrested decay due to 

 deficient rainfall. It has since been found, however, to be inde- 

 pendent of climate as in the same region some rocks may be found 

 deca_yed in this way while others in the immediate neighborhood 

 are completely decomposed. Curiously enough the soils in which 

 only a portion of the silicate elements of the original rock have 

 become earthy are considered very good and in Sao Paulo these 

 "rock-salt" {sabiiardo) soils are favorite ones with coffee planters. 



With the rocks thus far considered, that is to sa3% the crys- 

 talline and metamorphic schists and their associated granites, 

 decomposition, although very variable, is undoubtedly exten- 

 sive and; very probably, is in man}- points much more profound 

 than in anj' of those at which accurate measurements, or esti- 

 mates, have been made. In general, however, it is to be meas- 

 ured by scores and not by hundreds of feet and, so far as the 

 present evidence goes, the differences to be noted from one 

 point to another are rather to be attributed to original differ- 

 ences in susceptibility, in permeability (due to original or super- 

 induced textural and structural features), in the position of the 

 rock masses in their exposition to the agencies of decay or with 

 reference to the drainage level, rather than to climatic or biolosfic 

 causes. With reference to this last point it would be interest- 

 ing to compare the regions above considered with such a one as 

 that mentioned by Allen ^ in the semi-arid region of Central Bahia, 



' Hartt, Geol. and Phys. Geog. of Brazil, p. 314. 



