266 E. W. Hilgard — Peculiarities of Rock -Weathering. 



other easily obtainable fine powders have been similarly used. 

 Nor will the finest "slickens" of our quartz mills, which remains 

 suspended indefinitely in distilled water, serve for modeling, 

 any more than for pottery or porcelain. The utmost possible 

 comminution of graphite or talc, so closely similar to kaolinite 

 in softness and crystalline texture, will fail to impart to them 

 anything resembling the adhesive plasticity of colloidal clay. 



In the humid region the abundant and long-continued rains 

 cause the plastic clay in the surface soil to diffuse in the soil 

 water and to be carried into the subsoil, where much of it 

 stops at a shallow depth and thus produces the clayey subsoils 

 of humid regions. In the arid region, notwithstanding the 

 extreme fineness of many soils which causes them to rise as 

 dust at the slightest breath of wind, no such action occurs and 

 thus opportunity is afforded for the deep penetration of roots, 

 which allows much of the ordinary vegetation brought from 

 the humid region to do without rain or irrigation during a 

 six months drought. 



The fact that in most parts of the arid regions, the scanty rains 

 occur during the coldest portion of the year, probably accounts 

 in a large measure for the retardation of rock decay ; which, 

 others things being equal, is probably in all cases accelerated 

 by heat. On the other hand, the abrupt and constantly recur- 

 ring changes of temperature account for a large part of the 

 physical disintegration .which produces the great masses of 

 pulverized rock or sand which we find in "desert" regions. 

 The intense radiation of beat into space through the far 

 undersaturated air of the arid climates, which occurs so soon 

 as the sun disappears and often results in the violent disrup- 

 tion of cobbles from unequal contraction, naturally affects pro- 

 foundly the macrocrystalline rocks especially ; crumbling their 

 surface and enabling the torrents to carry off almost fresh u sand" 

 composed of all the mineral rock-constituents into the arid 

 valleys ; where under the influence of vegetation and increased 

 moisture their chemical decomposition can progress more 

 rapidly. 



JSTot, however, rapidly as in the humid region, where the 

 humus acids aid materially in mineral decomposition. For 

 not only is the humus-content of arid soils usually very much 

 less than in the humid, but so soon as formed such acids are 

 neutralized by the carbonate of lime always present in far 

 greater proportion than in the humid soils, save when the 

 latter are directly derived from calcareous formations. This 

 leads us to the consideration of chemical peculiarities of arid 

 and humid soils respectively. 



