CLIMATE AND TERRESTRIAL DEPOSITS 169 



MATURE TOPOGRAPHY AND VARIOUS CLIMATES 



Upon a mountain region becoming mature, its lower summits 

 and rounded slopes exercise less effect upon the climate, which now 

 becomes less accentuated and contrasted in nearby regions, and the 

 rainfall may extend for some distance beyond the crest line. Broad 

 extents of continental lands are here more important as modifiers 

 of the climates natural to the several zones. There is still a marked 

 relation in detail, however, between the climate and the surface. 

 The problem has been studied in Persia with particular reference 

 to climate by Huntington, who states: 



A prominent characteristic of the mature mountains of Persia is their naked- 

 ness, roughness, and steriUty. In a young country it is to be expected that there 

 shall be large areas of naked rock, but in a mature country, if the rainfall is 

 abundant, most of the surface, except the immediate valley sides, is graded, and 

 thus covered more or less deeply with soil. Eastern Persia, however, is so arid 

 that the ordinary state of affairs is reversed. All the mountains, whether young 

 or mature, are characterized by nakedness. Graded slopes are not a feature 

 of maturity in an arid climate.^ 



The valleys and basins are deeply filled with waste and the area 

 of exposed rock is not, however, so great as during the period of 

 youth. 



The differences in the chemical and physical composition and the 

 place of storage of the rock waste of arid and well-watered mountains 

 become emphasized with maturity. This heightened contrast is 

 due to the larger and deeper mantle of rock waste exposed to t he 

 particular climatic influences and moving more leisurely from its 

 original source to the reach of the streams. The contrast in kind of 

 products due to the climatic difference is well brought out by Hilgard,^ 

 who points out that — 



since kaolinization is also a process of hydration, the presence of water must 

 greatly influence its intensity, and especially the subsequent formation of colloidal 

 clay; so that rocks forming clay soils in the region of summer rains may in the 

 arid regions form merely pulverulent soil materials. Many striking examples 

 of these differences may be observed, e. g., in comparing the outcome of the 

 weathering of granitic rocks in the southern AUeghenies with that of the same 

 rocks in the Rocky Moimtains and westward, especially in California and Arizona. 



1 Explorations in Turkestan, Carnegie Institution Publications, No. 26, 1905, 

 pp. 247, 248. 



2 Soils, 1906, p."'47. 



