178 STUDIES FOR STUDENTS 



ground temperatures, the intensity of solar radiation being 26 per 

 cent, greater on the summit of Mount Blanc than at Paris.' Steep 

 rock faces favoring perpendicular action by the sun become of increas- 

 ing influence in the higher portions of the temperate zones and may even 

 in the arctic, as in Greenland, give a local importance to insolation as 

 a cause of rock destruction, von Drygalski having observed a tempera- 

 ture difference of 20° C. between the air and rock surface.^ 



In regard to the influence of atmospheric composition, it is known 

 that water vapor is the most efficient absorbing medium of the solar 

 heat and also prevents the rapid re-radiation of that part absorbed 

 by the earth's surface, preventing by both means high temperature 

 differences. Clouds also act both by preventing solar radiation from 

 reaching the ground and checking the escape of that already absorbed. 

 These direct effects, taken also in connection with the indirect effects 

 of the presence of water through vegetation, limit insolation as a mode 

 of rock destruction to mountain and desert regions. 



Finally, in considering the geological relations of climates to 

 erosion, possible variations of the solar constant of radiation must be 

 considered, an increased intensity of radiation, as previously pointed 

 out, increasing insolative rock disintegration, but only within the 

 limits given by the other conditions. The chief effects of such solar 

 variation would therefore be indirect, by changing the vapor content 

 and cloudiness of the atmosphere, both in geographic location and 

 amount. There are strong reasons for believing, though perhaps 

 hardly yet demonstrated, that an increase in solar radiation would 

 resuLt not only in a slight increase in the mean annual temperature 

 of the earth but also in areal increase of those portions of continental 

 interiors subject to arid and subarid climates. At the same time it 

 is probable that an expansion of the trade-wind belts into somewhat 

 higher latitudes would occur. Certain other regions would also be 

 marked by heavier rainfalls. On the whole, the result would be an 

 accentuation of climates and a marked increase in insolation as a 

 cause of rock disintegration. 



In conclusion, it is seen that the control of insolative rock disin- 

 tegration is largely geographic, being favored in past times by increase 



I Julius Hann, Handbook of Climatology, transl. by R. de C. Ward, p. 233. 

 =! Verdhaltniss Geschichte jiir Erdkunde, Berlin, 1891, p. 457. 



