I9I3.] OF GRANITES IN THE TROPICS. 169 



geneous rocks, the water running down the slopes sweeps along 

 mechanically the loosened fragments of the minerals just as it would 

 remove anything else. Channels are necessarily deepened most 

 rapidly where most water flows, and in the process of rock removal, 

 as a whole, the bottoms of these channels always keep in advance of 

 the divides between them. That is, it is a narrow localization of 

 mechanical action by water. 



It is especially noticeable that the fluting is a slow process, and 

 this slowness is explained by the fact that each furrow is made by 

 the little water that falls within its own narrow drainage basin. 



One may occasionally see in soft sandy clays furrows very 

 similar in form to the fluting of granites. There are some rather 

 striking examples in the San Bruno mountains just south of the 

 city of San Francisco in California. One of the accompanying 

 plates (Plate VIII.) shows two photographs of these particular 

 gullies. These photographs of gullies were made in January, 1912. 



The materials of these furrowed banks are sandy clays contain- 

 ing angular and subangular pebbles and rock fragments — apparently 

 soil and other products of the breaking down of the shales and sand- 

 stones of the San Bruno mountains. These materials also remind 

 one of the stone-capped earth-columns of the Tyrol, though in the 

 present case the stone caps are wanting. 



Such earth-columns are supposed to be the work of the mechan- 

 ical impact of rainwater. But most of the San Bruno furrows have 

 the appearance of being made, not necessarily by the impact of rain- 

 drops, but by the running down of the water that falls on and over 

 the furrowed surface. 



Water flowing down from the slopes above cuts the deeper gullies 

 in the face of the bank, but the minor trenches are supplied only by 

 water that falls on the immediate surface. 



The even spacing of these furrows is one of their striking fea- 

 tures. This is due I presume to the fact that, owing to the rather 

 even surface and the evenness of the water supply, there is usually 

 nothing to enable a channel to gain on its neighbors. 



