174 BRANNER— GRANITES IN THE TROPICS. [April 4 



and physical. As a whole the process is necessarily a slow one. The 

 localization of the run-off leads to an approximately even spacing of 

 the small streams and consequently to the even spacing of the fluting 

 where it appears over a broad surface. Somewhat similar erosion 

 forms are to be seen occasionally in homogeneous sandy clays, though 

 owing to the character of the materials the latter are cut rapidly. 



Though fluting seems to be confined to tropical countries, it is 

 worth noting that the temperature on the Serra do Itatiaya in Brazil, 

 where fluting is very marked, often falls below freezing. Evidently 

 some freezing does not interfere with fluting. 



Caldron-like pits are associated with fluting, and occur chiefly on 

 slopes not so steep as the fluted ones. They are most abundant on 

 the lower parts of the bare rock surfaces. They are formed by 

 water dissolving and disintegrating the minerals, and by the inflowing 

 waters mechanically stirring and floating the finer particles over the 

 rims of the basins. The chemical action of the water in the pits is 

 hastened by the decay of plants and other organisms that live and 

 die in the water left standing in the pits by the rain. 



Exfoliation is not a prominent feature of the fluted and pitted 

 masses. Indeed exfoliation hardly occurs at all in such places. Dis- 

 integration goes on rapidly, but it attacks the entire surface pretty 

 evenly. The feldspars seem to resist weathering better than the ac- 

 companying minerals, at least the feldspars are left standing out in 

 high relief over these surfaces. In time the mere heating and cooling 

 of the feldspars breaks them up, and they are washed oft' by the 

 torrential tropical rains as angular fragments or they are blown 

 off by the winds. 



The absence of talus about the bases of these fluted and pitted 

 hills is very striking. Indeed there are quite as many boulders on 

 the summits and sides of the hills as there are about their bases. 

 This seems to be due to the even attack of disintegration over all 

 surfaces, and to the fact that there is no freezing and thawing to chip 

 off the upper surfaces of hills and rocks and to pile up the fragments 

 at the bases of the slopes. 



Stanford University, Cal. 



