CLIMATE AND TERRESTRIAL DEPOSITS 171 



with stable precipitation but without glaciation has been considered 

 geologically equivalent to stable temperature and increased rainfall, 

 since the evaporation is decreased and the run-off consequently 

 increased, but an analysis of the problem would appear to show that 

 in its ultimate stratigraphic effects it is much more complex than this, 

 a complexity which has been recognized possibly for the first time 

 by Chamberlin and Salisbury,^ who state: 



The cold climate probably affected erosion, and therefore deposition in another 

 way, for the reduction of temperature was probably attended by a reduction of 

 vegetation, and any diminution of vegetation must have reflected itself in increased 

 erosion. The reduction of vegetation was probably greatest just where erosion 

 was most readily stimulated, namely, in the higher altitudes. The importance 

 of this consideration has perhaps not been duly considered. 



On the other hand, a marked rise in mean annual temperature 

 without change in precipitation will, in a region already under optimum 

 climatic conditions for vegetation, be equivalent to a movement toward 

 aridity. This is seen on comparing tropical deserts with those of 

 the temperate zone, where in the former a precipitation twice as 

 great may not prevent the existence of a similar aridity. The results 

 in the decrease of the vegetative hold upon the soil and a consequent 

 increased erosion, provided there is sufficient run-off to remove the 

 rock waste, will be similar to the results of a change toward unfavor- 

 able cold. 



It may be stated in conclusion, therefore, that any marked variation 

 of temperature away from that which in combination with the rainfall 

 gives the optimum conditions for vegetable growth will result in a 

 loosening of the soil and a corresponding increase in the rate of erosion. 

 The diminished area of rock covered by the soil and the thinner 

 covering where it does exist will, in combination with the lessened 

 amount of organic matter, result in an increase of disintegration and 

 a relative diminution of rock decay. 



EFFECTS OF INCREASED COLD 



On frost action and erosion. — Not only is the balance of the vegeta- 

 tive covering to erosive power disturbed, but during periods of 

 increased cold the frost action over exposed rock surfaces becomes 

 more energetic. Its intensity at high mountain elevations or in high 



I Geology, Vol. ITT, 1906, p. 453. 



