Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/66

 62 and the further addition of calarious earth produces a soil highly valuable to the agriculturist: 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 important office of producing food. Hence it happens that the great corn-fields, and the greatest population of the world, are placed on strata of the secondary and tertiary formations; or on their detritus, composing still more compound, and consequently more fertile diluvial, and alluvial deposites.

Another advantage in the disposition of stratified rocks consists in the fact that strata of limestone, sand, and sandstone which readily absorb water, alternate with beds of clay, or marl, which are impermeable to this most important fluid. All permeable strata receive rain-water at their surface, whence it descends until it is arrested by an impermeable subjacent bed of clay, causing it to accumulate throughout the lower region of each porous stratum, and to form extensive reservoirs, the overflowings of which on the sides of valleys constitute the ordinary supply of springs and rivers. These reservoirs are not only occasional crevices and caverns, but the entire space of all the small interstices of those lower parts of each permeable stratum, which are beneath the level of the nearest flowing springs. Hence if a well be sunk to the water-bearing level of any stratum,