SURFACE OF THE EARTH. Geology, by teaching us to look upon the form and distribution of land and sea, the features of hills and valleys, and the various deposits of peat, silt, gravel, &c., as effects of physical agencies, some of which are no longer in operation upon those areas where once they predominated, confers upon the surface of the earth an interest much greater than that which belongs merely to pictorial combinations, or even to agricultural utility and commercial adaptation. Uniformity, inequality, height, depth, and area, every the least peculiarity of form, whatever is remarkable in any part of the surface of the land or bed of the sea—these are effects of causes which require to be traced out before the problem of the physical history of the globe cau be considered as resolved. Geology was pro nounced by Sir C. Lycll, early in his career of research, to be the science of surfaces.
Superficial Deposits. If the stratified and unstratified rocks which compose the skeleton of the earth were laid bare to our view, the aspect of the globe would be far more rugged than it is now. The valleys would in many cast% lose their soft and easy curvatures and accordant slopes, in angular fractures and irregular chasms; the mountains and hills would lose those sloping buttress-like banks, com posed of fallen materials, which connect the broken ridges above with the level expanse below; a sterner aspect would belong to the now sinuous lines of seacoast; and an almost general barrenness would overspread the inlaod surface.
The eoil, gravel, clay peat, and other substances, which by their accumulation mask the features of the interior rocks, constitute a pecnllar class of phenomena which have been much, and yet not suffi ciently, studied by geologists. It is certain that without a more exact appreciation of the causes which have permitted the aggregation of the " superficial deposits " already named, our analysis of the processes whereby the earth has been made fit for the residence of man, and adapted to its present uses, must be very imperfect.
Soil is often supposed to be merely the disintegrated parts of the subjacent rocks, and this is sometimes really the case ; trap rocks, for example, of which the felspar and the hornblende become decomposed by the atmosphere, yield a soil often remarkable for fertility, and uncon taminated with foreign ingredients. But the soils which cover clays and limestones and sandstones are seldom of this simple origin. The bases of these soils may be generally derived from the subjacent strata, hut they usually contain foreign ingredients. The soil on the chalk and limestone hills of England is, often sandy, sandstones are covered by loam, and clays overspread with pebbles. The effect of this admixture of foreign substances with the disintegrated parts of the native rock is usually favourable to fertility.
We may often understand the cause of these admixtures by consider ing the effect of rains and currents of water on the sloping surface of the earth. These effects arrive at a maximum in particular vales and plains, into which many streams enter after flowing over strata of different kinds. In such vales the soil is in fact a mixture of calcareous, argillaceous, and arenaceous parts, and its indigenous plants are correspondingly varied, and include many which are not found growing together on any one of the soils which are here mixed together. [Sorb.] To watery agency, acting under the actual circumstances of physical geography, we may also ascribe many even extensive accumulations of gravel and sand which lie along the sides of valleys and in hollows of hills, or on the slopes of mountains; and it requires sometimes only the postulate, that in particular valleys inundations have formerly reached higher levels than at present, to apply the same explanation to terraces of gravel and sand now considerably above the actual flood-mark, but sloping parallel to the general inclination of the valley.