274 Mr. J. Y. Buchanan [May 8, 



It must be observed that, in the island of San Antonio, though 

 rain is rare moisture is abundant, and the valleys are fertile and well 

 cultivated. The moisture condensed on the ground, and retained 

 beneath the surface, aided by the high temperature due to the low 

 latitude, produces ideal conditions for the chemical decomposition of 

 the volcanic rocks of which the island consists, and the consequent 

 production of the best class of soil. This, owing to the rare occurrence 

 of rain, is allowed to remain in situ, and contributes to the maintenance 

 of a considerable population. The decomposition of the mineral con- 

 stituents, which yield to weathering and furnish the soil, undermines 

 the portions of the rock which resist it and removes their support. 

 These then surely gravitate 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 inter- 

 fered with by running water. 



Tlie^^ GrimibW Formation. — I never accepted with enthusiasm 

 the teachings of the ultraglacial school of geology, but it was only 

 during the course of the cruise of the " Challenger " 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 remain- 

 ing 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 con- 

 vincing evidence of the powerful decomposing action of the heat 

 and high vapour tension which characterise the tropical atmosphere. 

 Fig. 7 represents the Chilian town Antofagasta. The configuration 

 of the mountain slopes illustrates the effects of chemical and gravita- 

 tio'nal degradation in a rainless district. The landscape 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 con- 

 sideration 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 only be 

 responsible for clearing the debris out of the valleys and distributing 

 it over the plain, that the production of the debris itself might be 

 attributed to the occurrence of the last of the usually postulated warm 

 "inter-glacial periods, and that the existence of the debris furnished the 

 best evidence of the reality of a previous warm period. It is not neces- 

 sary for the cHmate during this period to have been anything like as 

 warm as that of the equator; all that is requu'ed is moderate warmth 

 and moisture. 



