CLIMATE AND TERRESTRIAL DEPOSITS 179 



of area and unification of the continental surfaces, giving rise to largely- 

 increased areas of continental climates. The indirect action of the 

 sun, however, as just pointed out, is probably an equally potent factor. 



According to Murray, one-fifth of the land surface is nov^ desert, 

 having no drainage to the sea.^ Over this region insolation and 

 eolian abrasion are the chief modes of rock destruction. It may well 

 have happened that in past times of wide epicontinental seas with 

 moist atmospheres and world-wide equable climates, or in times of 

 cold and glaciation, insolation may have sunk to half its present 

 importance as a mode of rock destruction. On the other hand, 

 times of broadened land areas, especially if occurring simultaneously 

 with high solar radiation, may have increased the desert areas beyond 

 their present extent, or at least shifted their limits into higher latitudes, 

 giving rise to peculiar characteristics, such as distinguish particularly 

 the Triassic formations. 



The first factor in the acquisition of any such distinctive characters 

 of sediments must depend upon the mode by which the parent rock 

 masses are destroyed. In this respect frost action and insolation act 

 alike, producing rock disintegration without rock decay, both most 

 efficient in regions without heavy precipitation and accentuated by 

 climatic movements away from a temperate condition, but in opposite 

 directions. The subsequent effects of these opposite climates upon 

 the sediments are, however, widely different, as will be made evident 

 on other pages. 



A conclusion of some stratigraphic importance is that, if the Gila 

 conglomerate has been correctly interpreted, it is seen that in desert 

 mountains strongly increased frost action with slightly decreased 

 insolation of glacial times is more effective in supplying waste than 

 the present slightly increased insolation with much weakened frost 

 action. 



Effects of increased heat on rock decay. — Rock decay implies the 

 presence of water, since it is only in its presence that not only hydration 

 but carbonation and oxidation of mineral substances can take place. 

 The problem, then, is in regard to the influence of temperature in 

 promoting rock decay in regions of moist climate. In regard to this 

 von Richthofen is one of the first to observe that — 

 » I "Origin and Character of the Sahara," Science, Vol. XVI (1890), p. 106. 



