Chapter XIII: 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_.

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.

5. Sedimentary rocks are often found with the planes of their strata more or less _inclined_. It is evident that they were not thus formed. The depositions of sediment from water will always be horizontal, or, at most, only slightly inclined. But there is often evidence in the rock itself that its strata were once horizontal. It is frequently observed that vertical strata contain pebbles with their longer axes in the plane of the strata. (Fig. 51.) When these pebbles were deposited, the longer axes would take, on an obvious mechanical principle, a horizontal position. Their present vertical position must have resulted from a change in the position of the strata in which they are enclosed. The same thing is shown by the position of a petrified forest in the south of England, known as the Portland dirt-bed. Some parts of it are inclined at an angle of forty-five degrees. The position of the vegetable remains (Fig. 52) shows that when they were growing the surface was horizontal.

The line _b d_ (Fig. 53), on inclined strata which makes with the horizon the greatest angle, is called the _direction of the dip_. The angle thus formed (_a b d_) is the _angle of inclination_. When inclined strata come to the surface, the exposed edge, _b c_, is the _outcrop_, and the line of outcrop on a horizontal surface is called the _strike_ of the strata.

When the inclined position is produced by an uplift of the strata, along a given line, so that they dip in opposite directions, this line is called an _anticlinal axis_, as at Fig. 54. If, however, the strata are fractured along this line, as at _b_, the fracture becomes a _valley of elevation_.

If depression take place along a given line, as at _c_, the strata will dip towards this line, and it will be a _synclinal axis_. The depression will be a _valley of subsidence_. A synclinal axis would also be produced by an elevation of the strata, as at _d_ and _e_, on each side of it, and the valley thus produced is one of elevation.

When successive sets of strata, as _f_ and _d_, Fig. 53, are not parallel, they are said to be _unconformable_.

6. When the strata are subjected to displacement, they do not always take a merely inclined position, but are often _contorted_ (Fig. 55), or folded together (Fig. 56). These _folded axes_ frequently succeed each other for many miles. (See Figs. 7 and 82.) In the case represented by Fig. 56, if the highest portion has been removed, so that the line _a b_ represents the actual surface, we shall have apparently a succession of deposits, of which those at _b_ would be the newest, and the oldest would be found at _a_, when in fact the strata at the extremities are parts of the same layer.

It is probable that disturbances like those now mentioned have been taking place continually, in different places, from the earliest times. There have been no periods of universal disturbance, and none of universal repose. On the contrary, the periods of disturbance in one part of the world have been periods of repose in another. For example, the coal measures of Europe were much broken and disturbed before the deposition of the new red sandstone, and the close of the coal period was at one time supposed to have been a period of general convulsion. It is now ascertained that the principal coal-fields in this country were not much disturbed at that period, and have not been since.