Geological _maps_ point out with more or less precision the extent of country over which the chalk, the red sandstone, the granites, &c., are found immediately beneath the loose materials on the surface; and these maps are of great value in indicating also the general quality of the soils over the same districts. It may be true, that here and there the _natural_ soils are masked or buried by transported materials, yet the _political economist_ may, nevertheless, with safety estimate the general agricultural capabilities and resources of a country by the study of its geological structure—the _capitalist_ judge in what part of it he is likely to meet with an agreeable investment—and the _practical farmer_ in what country he may expect to find land that will best reward his labours—that will admit of the kind of culture to which he is most accustomed, or, by the application of better methods, will manifest the greatest agricultural improvement. SECTION III.—OF THE PHYSICAL CHARACTERS OF SOILS. The influence of climate on the fertility of a soil is often very great. This influence depends very much upon what are called the _physical_ properties of soils. 1. Some soils are heavier and denser than others, sand and marls being the heaviest, and peaty soils the lightest. In reclaiming peat lands, it is found to be highly beneficial to increase their density by a covering of clay, sand, or limestone gravel. 2. Again, some soils absorb the rains that fall, and retain them in larger quantity and for a longer period than others. Strong clays absorb and retain nearly three times as much water as sandy soils do, while peaty soils absorb a still larger proportion. Hence the more frequent necessity for draining clayey than sandy soils; hence also the reason why, in peaty lands, the drains must be kept carefully open, in order that the access of springs and of other water from beneath, may be as much as possible prevented. 3. When dry weather comes, soils lose water by evaporation with different degrees of rapidity. In this way a siliceous sand will give off the same weight of water in the form of vapour, in one-third of the time necessary to evaporate it from a stiff clay, a peat, or a rich garden mould, when all are equally exposed to the air. Hence the reason why plants are so soon burned up in a sandy soil. Not only do such soils _retain_ less of the rain that falls, but that which is retained is also more speedily dissipated by evaporation. When rains abound, however, or in very moist seasons, these same properties of sandy soils enable them to sustain a luxuriant vegetation, when plants will perish on clay lands from excess of moisture.