I9I3.] OF GRANITES IN THE TROPICS. 165 



the miniature watersheds characteristic sharp, but somewhat ragged, 

 combs of the rock. Between these divides are narrow, round- 

 bottomed furrows or grooves that run down the slopes of the rocks 

 by the shortest routes. These shallow grooves suggest the marks 

 made by the fingers when drawn across a mass of plastic clay or 

 putty. The furrows or grooves in limestone, however, are, as a 

 rule, only an inch or so in diameter ; that is the fluting of limestones 

 is not usually on a large scale. 



The illustrations given in Dr. Stille's " Geologische Charakter- 

 bilder," Heft lo, however, show flutings in limestones of various 

 kinds, and some of these have unusually large furrows. 



The fluting of granites and other coarsely crystalline rocks, how- 

 ever, is on a large scale, and the grooves have only a remote resem- 

 blance to those on limestone surfaces. The fluted surfaces neces- 

 sarily appear only where the rock is entirely bare of soil. For the 

 mos't part the furrows start at the summit of the exposed rock or as 

 near it as possible, and run straight down the rock slopes by the 

 shortest possible routes. Those seen at and about Quixada reach a 

 maximum depth of nearly two meters measured at right angles to the 

 general surface of the rock masses. This takes no account of the 

 ordinary gullies cut by the larger streams. Instead of having sharp 

 combs separating the drainage areas of the different furrows, the 

 divides or miniature watersheds on the granite surfaces are always 

 rounded. But while the surfaces of the granite rocks are rounded in 

 general outline, they are quite rough, this roughness being caused by 

 the coarse crystals standing out boldly over the entire exposed rock 

 surface. About Quixada the rocks contain but little quartz, and 

 feldspars are the minerals that produce this roughness of surface. 



Caldron-like Pits. 



In the Quixada region the fluted rocks are covered here and there 

 with great rounded caldron-like pits some of which are associated 

 directly or indirectly with the fluting. These pits are shown in some 

 of the accompanying illustrations (Plates III., IV., V.). They are not 

 usually very deep, that is, they seldom exceed a depth of two meters 



