SECTION II.—CHANGES IN THE MASS OF THE STRATIFIED ROCKS.
1. The stratified rocks were deposited as mud or sand, and were at first in a yielding state. Most of these deposits have become solidified rock, such as limestone, clay slate and sandstone. The chalk of England is, however, but imperfectly consolidated, the great sandstone formation of New Holland is a friable mass easily disintegrated, and occasionally beds of clay in a plastic state are found as far down as the coal. Among the later rocks the solidification is less general, though there is some degree of hardening in all except the most superficial layers. The fissile structure results from the solidification of the particles composing each layer separately.
2. Since the solidification of the strata, or perhaps in connection with it, there has been something of movement among the particles, resulting in mineral veins, conchoidal structure, &c. One of the most general changes of this kind is that by which a mass becomes separable into thin sheets, independent of the stratification, and not parallel with it. This structure is represented by [Fig. 48], in which the heavier lines are those of stratification, and the lighter of cleavage.
3. The strata have been everywhere more or less broken, and the fractures, nearly vertical, extend to groat depths. When a fracture reaches the surface, it often becomes a channel for water. It is thus widened by the erosion, the deepest parts become filled with debris, and it becomes a gorge, ravine or valley.
If the fracture does not come to the surface, it becomes a cavern. In limestone, caverns which are formed in this way are very frequent, and extend for many miles. There is generally a stream of water running through them, but not of sufficient volume to have produced the erosion which has been effected.
When the sides of the fracture are but little separated, some mineral often separates itself from the adjacent rock, and filling up the space, reunites the broken parts. It is then called a vein of segregation ([Fig. 49], a b). But the fracture is more frequently filled with some volcanic rock injected from below. It is then a dike (c d), and may have a width of many rods, though it often diminishes in width till it is a mere thread. A dike of which the injected material is a metallic ore is a mineral vein.
