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.