ACTION OF WATER AKD ICE 275 



composed. This creeping process he ascribed wholly to the 

 expansive action of included water passing into the condition 

 of ice, the expansion taking place laterally and the material 

 being pushed down the slope along the line of least resistance. 

 Mr. C. Davidson has since taken up the subject experimentally; 



Fig. 24. — Showing direction and rate of motion of soilj tlie arrows showing, 

 hj their relative lengths, the rate of movement at various points, a, soil; 

 b, bedrock. 



and shown that the amount of the creeping could be accounted 

 for by the ordinary laws of gravity, the frost, by its expansion, 

 raising the individual particles a slight distance, and, on thaw- 

 ing, allowing them to drop back again a greater or less distance 

 down the slope, according to the angle of inclination. Dr. 

 Milton Whitney has, however, shown^ that there is an almost 

 continual movement among soil particles, dependent upon 

 meteorological conditions quite aside from those involved in 

 freezing and thawing. The creeping appears therefore to be 

 but the manifestation, in mass, of the inclination of each indi- 

 vidual particle to slide down the slope. 



The accumulations of talus at the foot of every cliff and on 

 the slopes of hills and mountains are matters of such every-day 

 observation as to need no mention in detail. 



(2) The Action of Water and Ice.^ — The power of a stream 

 to transport rock debris depends naturally upon its volume 

 and the rapidity of its current. This, on the supposition that 

 the character of the sediment to be transported remains the 

 same. According to the calculations of Hopkins, as quoted by 



^ Some Physical Properties of Soils, Bull. No. 4, U. S. Weather Bureau, 



1892. 



^Students are referred to Professor E. B. Salisbury's article on Agencies 



which Transport Material on the Earth's Surface, Journal of Geology, Vol. 



Ill, 1895, p. 70. 



