Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/65

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With regard to their adaptation to human uses, it may be stated generally, that the greater number of the most populous and highly civilized assemblages of mankind inhabit those portions of the earth which are composed of secondary and tertiary formations. Viewed, therefore, in their relations to that agricultural stage of human society in which man becomes established in a settled habitation, and applies his industry to till the earth, we find in these formations which have been accumulated in apparently accidental succession, an arrangement highly advantageous to the cultivation of their surface. The movements of the waters, by which the materials of strata have been transported to their present place, have caused them to be intermixed in such manner, and in such proportions, as are in various degrees favourable to the growth of the different vegetable productions, which man requires 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 maintenance of vegetation, by simple exposure to atmospheric agency; the disintegration produced 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 ingredients.

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 comparatively barren: the admixture of a small proportion of clay gives tenacity and fertility to sand,