CLIMATE AND TERRESTRIAL DEPOSITS 177 



of temperature must occur, and the effects will be proportional to the 

 magnitude and rapidity of these changes. As causes tending to 

 magnify these rapid temperature variations may be cited: first, a 

 short transmission of the sun's rays through the atmosphere, implying 

 a thin atmosphere as found on mountains and high plateaus or a high 

 altitude of the sun; second, surfaces at right angles or nearly so to 

 the sun's rays; third, a lack of clouds or of water vapor and to a 

 lesser extent a lack of carbon dioxide in the atmosphere, the former 

 characteristic of continental interiors, the latter of certain geological 

 epochs; fourth, a high value of the solar radiation will increase the 

 disintegrative effects by producing more rapid heating and, as a 

 result of the higher surface temperatures attained, a more rapid 

 cooling when the rock surface passes into shade. Sudden dashes of 

 rain, such as are characteristic of arid and semiarid regions, also 

 operate as a powerful cause of rock disintegration. 



To take up these in order : Angot has shown that, although at 

 the summer solstice the quantity of heat received per day at the poles 

 is greater than at the equator, if o . 2 of the solar radiation is absorbed 

 by passing vertically through the atmosphere, then at the poles less 

 reaches the surface of the ground than at the equator.' The con- 

 stancy of the polar daylight at the solstice tends also to prevent rapid 

 temperature changes. 



The daily maximum insolation of the surface is found to be not far 

 from a constant quantity up to lat. 60, with a maximum at from 30 

 to 40 degrees. In the higher latitudes, however, this maximum insola- 

 tion occurring at the summer solstice lasts for but a short time, sinking 

 in the winter to an insignificant quantity. It may be stated then on 

 theoretical grounds that, other conditions beiag favorable, thermal 

 disintegration may operate strongly to the limits of the polar zones, but 

 that the aggregate effect varies approximately with the latitude, 

 reaching a maximum at the equator and disappearing as an important 

 factor on the polar circles. 



Mountain elevations exert an influence upon insolation as impor- 

 tant as latitude, a marked difference being noted between the air and 



I Alfred Angot, "Recherches theoretiques sur la distribution de la chaleur a la 

 surface du Globe," Ann. bur. central met. de France, Tome I, 1883, Paris, 1885, 

 B 121-B 169. 



