DECOMPOSITION OF ROCKS IN BRAZIL 53 I 



the Cornish granites frequently extends to a depth of 50 or 60 

 feet, which represents fairly well the generality of cases of decay 

 of this t\-pe of rock in Brazil. 



Although not clearl}' expressed, it is evident that most 

 writers in treating of the region about Rio de Janeiro, to which 

 most of the observations on rock decay in Brazil refer, have 

 assumed that the massive types of granitoid gneiss and, to a 

 subordinate extent, of true granite, which are about the only 

 rocks seen in a sound condition, represent the generality of 

 rock in the region, and that the great masses of decomposed 

 rock seen belong necessarily to these resistant types. It seems 

 also to have been assumed that the accidents of relief giving 

 abrupt differences of level of hundreds, or even thousands, of 

 feet are mainlv due to extraordinary erosion preceded by decay 

 to extraordinarv depths. The clearest expression of this view is 

 found in the long-shot explanation by Agassiz of the so-called 

 organpipe peaks (see Fig. 4 of Branner's paper) as hard vertical 

 strata of gneiss left standing by the gradual decay of softer 

 intervening strata. Up to the present time the peaks in question 

 have never been examined close at hand by a competent geol- 

 ogist and as both gneiss and granite occur prominently in the 

 range it is doubtful which of these two types of rock forms the 

 peaks, or what may occur in the intervals between them. 



It is certain, however, that the apparent uniformity of the 

 massive t_ypes of rocks is due to their great resistance to decay 

 in virtue of which they form the greater part of the hills which 

 generally present bare rocky bosses at the summit while the 

 flanks are heavily wooded. In the flanks between these hills 

 basic eruptives, principally diabases, frequently appear which, 

 though equally massive, seem to be more susceptible to decay 

 although their apparent limitation to the lower levels ma}" in 

 part be due to their emergence along lines of fracture and of 

 consequent weakness and of susceptibility to decay. Much 

 more frequent in the lower grounds and in the hills of deeply 

 decomposed material are schistose gneisses which from their lack 

 of homogeneity and their position with the planes of fissibility 



