276 Ice and its Natural History 



class of soil. This, owing to the rare occurrence of rain, is 

 allowed to remain in situ, and contributes to the main- 

 tenance of a considerable population. The decomposition 

 of the mineral constituents, which yield to weathering and 

 furnish the soil, undermines the portions of the rock which 

 resist it and removes their support. These then surely gravi- 

 tate to the lowest level, where their weathering is continued, 

 causing always more and more perfect rounding of the stones, 

 with the production of the equivalent amount of soil in which 

 the stones remain embedded, provided that the process is not 

 interfered with by running water. 



The " Crumble " Formation. I never accepted with en- 

 thusiasm the teachings of the ultraglacial school of geology, 

 but it was only during the course of the cruise of the " Chal- 

 lenger" that I became convinced that ice is not required 

 to produce denudation. I found in all countries within the 

 tropics that the rocks were decomposed to a depth of 20 or 

 30 metres, the resulting material often remaining in situ with 

 such a fresh appearance that it was difficult to imagine that 

 it could be anything but the unaltered rock. 



It was only necessary, however, to touch it with a stick, 

 or even with the fingers, for it to crumble into fragments of 

 all sizes down to sand and clay. In my journal almost every 

 new island or place visited is logged as consisting of " the 

 usual crumble formation." There could be no doubt about 

 the cause of it. It furnished convincing evidence of the 

 powerful decomposing action of the heat and high vapour 

 tension which characterise the tropical atmosphere. Fig. 7 

 represents the Chilian town Autofagasta. The configuration 

 of the mountain slopes illustrates the effects of chemical and 

 gravitational degradations in a rainless district. The land- 

 scape so produced consists of a succession of taluses. The 

 debris thus accumulated on the mountain side would be 

 cleared out if it was the side of a valley and a glacier passed 

 along it. Applying this knowledge to the consideration of 

 the conditions in my own country, which were referred to ice 

 action, such as the enormous quantities of gravel in the Spey 

 valley, I came then to the conclusion that the glaciers could 



