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_. [Illustration: Fig. 48.] 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_. [Illustration: Fig. 49.] 4. The uplifting force by which the fracture is produced has frequently raised the rock on one side higher than it has on the other. This is called a _fault_. (Fig. 50.) The unequal movements by which the fault is produced seem in some instances to have been repeated several times, and the grinding of the broken edges upon each other has polished and striated the sides of the fracture. [Illustration: Fig. 50.] [Illustration: Fig. 51.] [Illustration: Fig. 52.]