280 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EAUTH. 



materials from the surface layer must ultimately progress further for 

 a given advance of disintegration than if the rock rapidly yields to the 

 actions tending to disintegrate it. 



In the surface layer the rain charged with carbonic and humic acids 

 principally exerts its effects, the more soluble constituents yielding, of 

 course, before the less soluble, and so growing finer in grain as time 

 progresses. The more soluble substances thus become concentrated 

 in the liner constituents of the soil.^ 



Ultimately, if mechanically transported to the rivers, a sorting, 

 according to mass and dimensions, occurs. The liner-grained particles 

 are carried on a current which drops the coarser particles. Thus the 

 finer silts are richest in the soluble constituents of the former soils. 

 They constitute material on which vegetation flourishes, and if deposited 

 in the ocean build up rock masses rich in alkalies, chiefly, as we have 

 seen, in potash. Nearer the shore the coarse grits and sandstones, 

 poor in alkalies, accumulate. 



Subsequent upheaval brings to the surface rocks, of which the 

 finer-grained and softer varieties are those possessing the larger share 

 of alkalies. These generally, owing to secondary or in some cases 

 prinuuT mica and their fineness of grain, arc most distinctly cleavable. 

 Such slates contain from 3 to 5 per cent of alkalies. 



The dissolved materials pass through a difi^erent history, but in the 

 limestones, etc., to which they^ give rise most generally there exists 

 an amount of detrital feldspathic matter sufficient, when again uplifted 

 and weathered, to yield soils scarcely less rich in alkalies than those 

 derived directly from the parent rock. 



This last fact is one of great interest. Merrill shows that soils 

 derived as residual material from the most diverse rocks are very 

 similar in composition. 



The full tables should be consulted.'^ 



Residual soil from limestone, Wisconsin, 4.5 feet from surface . 



Same, Si feet from surface 



Residual soil from limestone, Wisconsin, 3 feet from surface. . . 



Same, 4 J feet from surface 



From dolomite, Alabama 



From diabase dike. North Carolina 



A gabbro soil, Maryland 



Subsoil from Trenton limestone, Maryland 



Soil from Triassic sandstone, Maryland 



Trenton limestone, unaltered 



Residual soil from same 



Gneiss, Virginia 



Soil from same 



Diorite, Virginia 



Soil from same 



1 See Eocks, Rock Weathering, and Soils, pp. 365, 366, where this is proved by 

 mechanical and chemical analysis. 



'Loc. cit., pp. 305, 306; also, pp. 358, 359. 



