Ice and its Natural History 277 



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 interglacial periods, and that 

 the existence of the debris furnished the best evidence of the 

 reality of a previous warm period. It is not necessary for 

 the climate during this period to have been anything like as 

 warm as that of the equator ; all that is required is moderate 

 warmth and moisture. 



I have attributed the presence of fragments of rock, 

 whether great or small, and to a considerable extent their 

 rounded form, to the chemical action of the moisture derived 

 from the atmosphere. Water penetrates, by its own weight, 

 into any cracks or joints which may occur in the rock, 

 and into minute discontinuities of substance by capillarity. 

 It spreads and extends its decomposing influence along the 

 surfaces which lend themselves most readily to the process 

 of soaking. The warmer the rock is, the more energetic is 

 the decomposing action. On the tops of high mountains rock 

 surfaces, exposed to the direct rays of the sun, acquire often, 

 even in winter, a high temperature, and when they are covered 

 by snow, they are protected from excessive cooling by 

 radiation. All rocks have surfaces of relatively imperfect 

 continuity. These are found by water, which enters easily 

 provided the temperature of the surface of the rock is not 

 such as to convert it into ice. Far-spreading decomposition 

 is then only a question of time, and, after it has spread the 

 falling asunder of the parts of the rock under the influence of 

 gravity is a certainty. There is no necessity for assistance 

 by any other agency. The chemical action of atmo- 

 spheric moisture and the tendency of every part of a 

 mountain or rock to yield to gravity when not adequately 

 supported, suffice to account for all the degradation of 

 rock which we observe. 



I have not been able to discover the author of it, but it is 

 a very old and generally accepted doctrine, that the rock 

 fragments which are found so frequently covering the tops of 

 mountains are split off from the parent rock by the energy 



