ADAPTATION TO AGRICULTURE. 69 
such manner, and in such proportions, as are in 
various degrees favourable to the growth of the 
different vegetable productions, which man re- 
quires for himself and the domestic animals he 
has collected around him. 
The process is obvious whereby even solid 
rocks are converted into soil fit for the main- 
tenance of vegetation, by simple exposure to 
atmospheric agency ; the disintegration pro- 
duced by the vicissitudes of heat and cold, 
moisture and dryness, reduces the surface of 
almost all strata to a comminuted state of soil, 
or mould, the fertility of which is usually in 
proportion to the compound nature of its ingre- 
dients. 
The three principal materials of all strata are 
the earths of flint, clay, and lime ; each of these, 
taken singly and in a state of purity, is com- 
paratively barren : the admixture of a small 
proportion of clay gives tenacity and fertility to 
sand, and the further addition of calcareous 
earth produces a soil highly valuable to the agri- 
culturist : and where the natural proportions are 
not adjusted in the most beneficial manner, the 
facilities afforded by the frequent juxta-position 
of lime, or marl, or gypsum, for the artificial 
improvement of those soils which are defective 
in these ingredients, add materially to the 
earth's capability of adaptation to the impor- 
tant office of producing food. Hence it happens 
