19I3-] OF GRANITES IN THE TROPICS. 173 



drons several times without there being enough to answer the pur- 

 poses of the planters and cattle growers. 



It may be worth noting also that in dry seasons, and as long as 

 the water lasts, birds and other animals flock to these caldrons to 

 drink. 



The total result of the alternation of rain and drouth must be a 

 frequent filling and emptying of the pits, a frequent stirring up of 

 the fine materials in the bottoms of the pits, and its removal in 

 mechanical suspension by the water overflowing the lips of the 

 caldrons. 



These facts also suggest why the caldrons are most abundant 

 about the lower slopes rather than on the crests of the hills : there is 

 a larger run-ofif, and consequently a more frequent and a more 

 vigorous disturbance of the water in the pits. 



The horizontal pits occasionally seen in vertical rock walls are 

 evidently not made in the same way as the vertical pits formed 

 in flat or sloping surfaces. The latter seem to be due to the 

 alternate absorption and evaporation of water much as fret- work 

 is formed over the surfaces of porous sandstones by the prying 

 off of sandgrains. Some remarkable cases of this kind are known in 

 the interior of the state of Bahia in Brazil. Instances of this sort are 

 now being studied and no further mention of them need be made at 

 present. 



Conclusions 



The fluting and pitting of coarse grained crystalline rocks appears 

 to be confined to tropical countries, and to massive, homogeneous 

 rocks openly exposed. 



The grooves that make up the fluted surfaces run down the rock 

 faces by the shortest possible courses, and are made by the small 

 amount of water that falls upon and flows down the fluted surfaces 

 themselves. In other words there are no strong streams flowing 

 across fluted surfaces whose waters are gathered over a wide area. 



Fluting seems to be confined to steep slopes. The angle of such 

 slopes cannot be stated, but in the cases observed it usually was 

 forty-five degrees or more. 



The process of fluting is partly chemical and partly mechanical 



