In the preceding chapter we have studied the rocks which make up the crust of the earth, so far as that is accessible to observation. It remains for us to inquire how these rocks are arranged on a large scale, and to what displacements and dislocations they have been subjected since the time of their formation. Examined with reference to the simplest and broadest facts of structure, we find that rock masses fall into two categories: (1) Stratified Rocks, and (2) Unstratified or Massive Rocks. A very brief examination will show us that these two categories correspond respectively to the sedimentary and igneous divisions of the classification according to mode of origin, neglecting, for the present, the metamorphic class.
We shall begin our study of rock masses with the stratified series, because their structure and mode of occurrence are, on the whole, the simplest and most intelligible, and tell their own story. The unstratified series, on the other hand, can be understood only by determining their relation to the former.
The stratified rocks form more than nine-tenths of the earth's surface, and if the entire series of them were present at any one place, they would have a maximum thickness of about thirty miles, but no such place is known. The regions of greatest sedimentary accumulation are the shallower parts of the oceans, while those regions which have remained as dry land, through long ages, may not only have had no important additions to their surfaces, but have lost immense thicknesses of rock through denudation. The great oceanic abysses are also areas of excessively slow sedimentation, and thus the thickness of the stratified rocks varies much from point to point, a variation which has been increased by the irregularities of upheaval and depression and of different rates of denudation. Even with this irregularity in the formation and removal of the stratified rocks, it would be exceedingly difficult, if not impossible, to investigate the entire series of them, if they had all retained the original horizontal positions in which they were first laid down.
In many places, however, the rocks have been steeply tilted and then truncated by erosion, so that their edges form the surface of the ground, and thus great thicknesses of them may be examined without descending below the surface.
Stratification, or division into layers, is the most persistent and conspicuous characteristic of the sedimentary rocks. In studying the sedimentary deposits of the present day (Chapter VII (Reconstructive Processes. - Continental Deposits, Land, Swamp, And River)) we learned that by the sorting power of water and wind, heterogeneous material is arranged into more or less homogeneous beds, separated from one another by distinct planes of division, and the same thing is true of the sedimentary rocks of all ages. This division into more or less parallel layers is called stratification, and the extent to which the division is carried varies according to circumstances.
A single member, or bed, of a stratified rock, whether thick or thin, is called a layer, though for purposes of distinction, excessively thin layers are called lamince. Each layer or lamina represents an uninterrupted deposition of material, while the divisions between them, or bedding planes, are due to longer or shorter pauses in the process, or to a change, if only in a film, of the material deposited. A stratum is the collection of layers of the same mineral substance, which occur together and may consist of one or many layers. However, the term is not always employed in just this sense and often means the same as layer. The passage from one stratum to another is generally abrupt and indicates a change in the circumstances of deposition, either in the depth of water, or in the character of the material brought to a given spot, or m both. So long as conditions remain the same, the same kind of material will accumulate over a given area, and thus immense thicknesses of similar material may be formed.
To keep up such equality of conditions, the depth of water must remain constant, and hence the bottom must subside as rapidly as the sediment accumulates.
Usually, a section of thick rock masses shows continual change of material at different levels. Figure 149 is a section of the rocks in Beaver County, Pennsylvania, in which several different kinds of beds register the changes in the physical geography of that area. At the bottom of the section is a coal seam (No. 1), the consolidated and carbonized vegetable matter which accumulated in an ancient freshwater swamp. Next came a subsidence of the swamp, allowing water to flow in, in which were laid down mixed sands and gravels (No. 2). The accumulations eventually shoaled the water and enabled a second peat swamp to establish itself; this is registered in the second coal bed (No. 3), the thinness of which indicates that the second swamp did not last so long as the first. Renewed subsidence again flooded the bog, as is shown by the stratum of shale (No. 4) which overlies the second coal bed. Next, the water was shoaled by an upheaval, and argillaceous sands were laid down, which now form the flaggy sandstones (No. 5) overlying the shale.
The twenty-five feet of sandstone, aided by continued slow rise, silted up the water and allowed a third peat bog to grow, the result of which is the third coal seam (No. 6), while a repetition of the subsidence once more brought in the water, in which were laid down the seventy feet of gravel at the top of the section. In this fashion the succession of strata records the changes which were in progress while those strata were forming. Whether the beds, other than the coal seams, were laid down in fresh water, or in salt, by a lake, a flooded river, or the sea, may be determined from the fossils contained in those beds. In the absence of fossils it is not always possible to make the distinction.
Fig. 149. - Section in coal measures of western Pennsylvania. (White).
Somewhat similar changes in the strata may be occasioned by the steady lowering of a land surface through denudation. This diminishes the velocity of the streams, which, in its turn, changes the character of the materials which the rivers bring to the sea.
We have no trustworthy means of judging how long a time was required for the formation of any given stratum or series of strata, but it is clear that different kinds of beds accumulate at very different rates. The coarser materials, like conglomerates and sandstones, were piled up much more rapidly than the shales and limestones; so that equal thicknesses of different kinds of strata imply great differences in the time required to form them. Comparing like strata with like, we may say that the thickness of a group of rocks is a rough measure of the time involved in their formation, and that very thick masses imply a very long lapse of time, but we cannot infer the number of years or centuries or millennia required.
Geological chronology can be relative only. Such a relative chronology is given in the section that we have examined by the order of succession of the beds. Obviously the lowest stratum is the oldest and the one at the top the newest. This may be put as a general principle, that, unless strata have lost their original position through disturbance or dislocation, their order of superposition is their order of relative age. It is for this reason that in geological sections the strata are numbered and read from below upward.
Change in the character of the strata takes place not only vertically, but also horizontally, since no stratum is universal, even for a single continent. Our study of the processes of sedimentation which are now at work, showed us that the character of the bottom in the ocean or in lakes is subject to frequent changes, varying with the depth of water and other factors. The same is true of the ancient sea and lake bottoms, now represented by the stratified rocks of the land. Strata may persist with great evenness and uniform thickness over vast areas, and in such cases the bedding planes remain sensibly parallel. But sooner or later, the beds, whenever they can be traced far enough, are found to thin out to edges and to dovetail in with beds of a different character. When the strata are of constant thickness for considerable distances, and the bedding planes remain parallel, the stratification is said to be regular. In many cases these changes take place rapidly from point to point, and then the strata are plainly of lenticular shape, thickest in the middle, thinning quickly to the edges.
Here the bedding planes are distinctly not parallel, and the stratification is irregular.
An example of rapid horizontal changes is given in the two accompanying parallel sections (Fig. 150), taken through the same beds, only twenty feet apart. In these sections the differences of thickness of the coal seams and of the sands and clays which separate them are very striking.
Fig. 150. - Parallel sections near Colorado Springs, Col. (Hay-den).
The finer details of structure of the stratified rocks, such as cross-bedding, ripple and rill-marks, rain-prints, tracks of animals, and the like, likewise afford valuable testimony as to the circumstances under which the rocks were laid down.