176 STUDIES FOR STUDENTS 



and the lack of evidence of highly increased precipitation at the time 

 would seem to be due more largely to frost action. 



In conclusion it would appear that where the chief effect of increased 

 cold is an increase of snowfall the change results in an increase in the 

 ratio of transportation to erosion, extending to the limits of such 

 increased snowfall, and not as in the case of increased frost action to 

 an increase of erosion over transportation. The absolute value of 

 both erosion and transportation may increase in both cases. 



EFFECTS OF INCREASED HEAT ON ROCK DISINTEGRATION AND 



DECAY 



It is seen that rock disintegration or physical weathering is at a 

 maximum in regions of exposed rock surfaces, while rock decay is 

 pronounced, on the contrary, where the solid rock is protected from 

 physical changes by the interposition of a layer of soil. The former 

 conditions of bare surfaces are found in mountainous regions, where 

 the steep slopes prevent the retention of soil; or in the deserts, where 

 the absence of ground-water prevents either the formation of an 

 effective vegetable covering or the carrying-forward of the chemical 

 processes of rock decay. The contrary conditions of soil-mantled 

 surface exist where the slopes are moderate or the presence of ground- 

 water gives rise to a vegetable covering sufficient to hold the soil and 

 diminish erosion of its upper surface to a rate equal to that at which 

 decay contributes to the lower strata of the soil. In a climate where 

 rock decay operates strongly there thus arises a very deep soil, partially 

 protecting the deeply buried solid rock from further decomposition and 

 slowing down the rate of decay till it equals the rate of surface ablation, 

 hindered in turn by the luxuriant vegetation also existing in such a 

 region. Both the topographic and climatic factors are thus seen to 

 be fundamental in the result. Under the present topic it is desired 

 to note what effects a climatic variation toward an increase of tem- 

 perature will have upon these processes of physical and chemical 

 rock destruction which necessarily precede subaerial denudation and 

 are the supplying agents for the diverse materials of sedimentary 

 formations. 



Effects on rock disintegration. — To produce rock disintegration a 

 high temperature is not necessary but diurnal or hourly variations 



