56 Joty—An Estimate of the Geological Age of the Earth. 
Chemical Denudation.—Quite another factor in the uniformity of solvent denu- 
dation is the chemical and physical nature of the rock surfaces and soils exposed 
during the successive ages of the Earth’s history. With reference to the view 
that in earlier times larger areas of igneous rocks were exposed to denudation 
than in more modern periods, some remarks on soil and weathering are 
necessary. 
We see in soil-formation of the present day a process of ever deepening disin- 
tegration of the parent rock, and simultaneously progressing decomposition of 
the upper layers. ‘This results in a surface layer, possessing a reduced percentage 
of the more soluble materials, which protects the richer material beneath. If the 
rock itself is for physical or chemical reasons highly resisting, the leaching out 
of soluble 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 princi- 
pally 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 finer constituents of the 
soil.* 
Ultimately, if mechanically transported to the rivers, a sorting according to 
mass and dimensions occurs. ‘The finer 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 sand- 
stones, 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, primary mica, and their fineness 
of grain, are most distinctly cleavable. Such Slates contain from 8 to 5 per cent. 
of alkalies. 
The dissolved materials pass through a different history, but in the lime- 
stones, &c., to which they give rise, most generally there exists an amount of detrital 
felspathic 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 
* See “ Rocks, Rock-weathering, and Soils,” pp. 865, 366, where this is proved by mechanical and 
chemical analysis. 
