532 ORVILLE A. DERBY 



standing nearly vertically are higlily permeable and so subject 

 to decay that they are rarely found in a sound condition and 

 have generally been overlooked. In the absence of a detailed 

 geological study it is impossible to estimate, even approx- 

 imately, the relative areas occupied by the comparatively 

 impermeable granitoid gneisses and granites and these highly 

 permeable schistose gneisses, but presumably the areas formerly 

 occupied by the latter were at least equal to those of the former. 

 As regards the origin of the present abrupt topographical 

 features the importance of faulting in their formation has 

 scarcely been hinted at, and yet, as an examination of the 

 excellent figures accompanying the paper cited will show, it 

 seems to afford the most natural explanation of man)' of these 

 features. Admitting that faulting on an extensive scale has 

 taken place in the region, it will be readily seen that the down- 

 throw side of a fault will be in very different conditions as 

 regards the agencies of decomposition (even when the same 

 type of rock is concerned) from the upthrow side. The latter 

 receives only the water that actually falls upon it from the clouds 

 and the time for infiltration (until a covering of decomposed 

 material is formed that serves to retain a portion of the water) 

 is practically limited by the duration of the rain. Under these 

 circumstances the granitoid gneisses and granites of Rio de 

 Janeiro and of the Serra do Mar region generally, are so nearly 

 impermeable that the tops of the bosses are almost always bare 

 and waste mainly by the process of exfoliation. On the slopes 

 where the rock may be more shattered and where talus accumu- 

 lates the meteoric waters have a greater scope for action partic- 

 ularly after vegetation has established itself on the decomposition 

 crust that is gradually formed which, serving as a saturated 

 sponge, greatly prolongs the time of infiltration of the surface 

 waters to the underlying rocks. The downthrust side of a fault 

 in addition to its own proper share of meteoric waters receives 

 also a large part of the drainage of the upthrust portion not only 

 during the downpour of rain but for some time after and the 

 access of the water to the interior of the rock mass will be 



