332 HOW CROPS FEED. 
large a part of the Middle States, but most of the rocks 
of New England have been soil, and have supported vege- 
table and animal life, as is proved by the fossil relics that 
have been disinterred from them. 
We have explained the agencies, mechanical and chemi- 
cal, by which dur soils have been formed and are forming 
from the rocks. By a reverse metamorphosis, involving 
also the coéperation of mechanical and chemical and even 
of vital influences, the soils of earlier ages have been so- 
lidified and cemented to our rocks. Nor, indeed, is this 
process of rock-making brought to a conclusion. It is 
going on at the present day on a stupendous scale in vari- 
ous parts of the world, as the observations of geologists 
abundantly demonstrate. If we moisten sand with a so- 
lution of silicate of soda or silicate of potash, and then 
drench it with chloride of calcium, it shortly hardens to 
a rock-like mass, possessing enough firmness to answer 
many building purposes (Ransome’s artificial stone). A 
mixture of lime, sand, and water, slowly acquires a simi- 
lar hardness. Many clay-limestones yield, on calcination, 
a material (water-lime cement) which hardens speedily, 
even under water, and becomes, to all intents, a rock. 
Analogous changes proceed in the soil itself. Hard pan, 
which forms at the plow-sole in cultivated fields, and 
moor-bed pan, which makes a peat basin impervious to 
water in beds of sand and gravel, are of the same nature. 
The bonds which hold together the elements of feldspar, 
of mica, of a zeolite, or of slate, may be indeed loosened 
and overcome by a superior force, but they are not de- 
stroyed, and reassert their power when the proper cir- 
cumstances concur. The disintegration of rock into soil 
is, for the most part, a slow and unnoticed change. So, 
too, is the reversion of soil to rock, but it nevertheless 
goes on. The cultivable surface of the earth is, however, 
on the whole, far more favorable to disintegration than 
to petrifaction. Nevertheless, the chemical affinities and 
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