Section Iv.--Changes on the Surface of the Earth.

1. The principal changes of this class consist in the wearing down and removing immense quantities of the surface rock. The form in which the igneous rocks, of which the entire crust of the earth was originally composed, now appear, furnishes no assistance in judging of the amount of denudation which they have suffered. We can judge only from the amount of rock for which they have furnished the materials, and these are the whole sedimentary series which exist both as dry land and as the bed of the sea.

2. The sedimentary rocks have also been subject to great denudation; and we often have, in what is left, some indications of how much has been removed. One of these indications consists in the now level surface of those portions of country in which large faults exist. By the excavations for coal, in England, faults have been discovered of five or six hundred feet. At the time that they were formed, the surface must have presented precipitous escarpments (as represented by the dotted lines in Fig. 50) of a height equal to the dislocation; but the whole is now reduced to a general level (z z), denuding causes having removed the elevated portions.

The extent of valleys will often give some idea of the amount of denudation to which a region has been subjected. In the north-west of Scotland there is a succession of hills of about three thousand feet in elevation, consisting, for the upper two thousand feet, of horizontal strata of old red sandstone. (Fig. 66.) We cannot conceive that these mountain masses were deposited in their present isolated form. The whole intervening spaces must have been filled with strata continuous with those by which the elevations are formed.

"I entertain little doubt that when this loftier portion of Scotland, including the entire Highlands, first presented its broad back over the waves, the upper surface consisted exclusively, from one extremity to the other, of a continuous tract of old red sandstone; though, ere the land finally emerged, the ocean currents of ages had swept it away, all except in the lower and last raised borders, and in detached localities where it still remains, as in the pyramidal hills of Western Rosshire, to show the amazing depth to which it had once overlaid the inferior rocks."--Miller, Old Red Sandstone, p. 22.

A somewhat similar instance occurs in the Connecticut river sandstone, in the central part of Massachusetts. The following figure (Fig. 67) represents two mountains of 1 the sandstone, between which the Connecticut river flows. The dotted lines indicate a depth of one thousand feet of the rock which has been swept away. It is also thought that a bed of equal depth has been removed from this section southward, through the State of Connecticut, to the sea-coast.

3. Valleys, and even many of the larger valleys, are produced by the wearing down of the surface. The lower portion of the Connecticut valley is one of denudation, though in its upper part it is a valley of elevation, resulting from the upheaval of the Green and White Mountains. The water-courses from the mountains are transverse to the direction of the ranges, and generally consist of valleys of denudation. These valleys were no doubt originally fractures, produced while the mountains were rising. The fractures have been subsequently widened by denudation into valleys.

4. The rocky surface, beyond the fortieth parallels of latitude, and in the vicinity of glacier-producing mountains, is generally covered with grooves and striæ (Fig. 68), varying from several inches in depth to the finest perceptible lines. Rocks that are of a soft consistence, or which have been long exposed to atmospheric agents, seldom exhibit these marks, though there are probably few places, outside of the parallels before mentioned, where the rocky surface, if it has been protected from atmospheric decay, does not contain such grooving.

5. Another change at the surface consists in the formation of a soil; that is, of a superficial layer, of no great thickness, of earthy matter, a large proportion of which is always in a minutely divided state. In some instances it is common sediment, unsolidified; in others, it consists of the surface rock in a state of disintegration; but a large part of the soil within the region where the grooved surfaces are found consists of materials transported from a distance.

Soils are distinguished according to their predominant minerals, as siliceous, aluminous and calcareous. If siliceous matter is in excess, it will be a light, warm soil, and allow the water to pass through it too freely. If the clay predominates, the soil is cold, stiff, and too retentive of moisture. A proper admixture of these three ingredients constitutes the best soils. There are some other mineral ingredients essential to the productiveness of soils, but they are always in small proportion. In addition to the inorganic part which is common to the upper soil, and the subsoil, there is required, in order to render the upper layer productive, a large admixture of decaying animal and vegetable matter.