DISINTEGRATION OF GRANITE. 315 



southern limits of the Serra do Mar do not appear to 

 be well-defined, but we may estimate its length at 

 from five to six hundred miles. The other mountain 

 systems of the empire are less well known ; but I 

 believe that the ranges dividing the province of Minas 

 Geraes from Goyaz, and the so-called Cordillera 

 Grande of the province of Goyaz, lying between the 

 two main branches of the great river Tocantins, are 

 largely formed of ancient sedimentary rocks of the 

 Laurentian and Huronian groups. 



The granite of the Serra da Mantiqueira and Serra 

 do Mar is coarse-grained, with large crystals of felspar, 

 and is therefore much exposed to disintegration. So 

 far as I know, the vast masses of detritus forming the 

 plateaux of this region show no other materials than 

 such as would be produced by the disintegration of 

 the crystalline rocks, and there is strong reason to 

 believe that these have never been overlaid by 

 sedimentary deposits. 



Let us now consider what must have been the past 

 history of a region formed of such materials, exposed, 

 during a large part of the past history of the earth, to 

 the action of the elements. In such an inquiry one 

 of the chief points for consideration is the amount of 

 rainfall. The direct effect, both mechanical and 

 chemical, of rain falling on a rock surface is perhaps 

 not the most important. Still more essential is its 

 action in removing the disintegrated matter, and 

 thereby exposing a fresh surface to renewed action. 

 The difference in the absolute result due to abundant 

 or deficient rainfall would be found, if we could cal- 

 culate it accurately, to be enormous. In a nearly 



