In consequence of the influence of time, weather, atmospheric action of all kinds, the apparent relation of beds has often become so completely reversed that it is exceedingly difficult to trace their original relation. Take, for instance, the following case. An eruption has upheaved the strata over a given surface in such a manner as to lift them into a mountain, cracking open the upper beds, but leaving the lower ones unbroken. We have then a valley on a mountain-summit between two crests resembling the one already shown in Figure 4. Such a narrow passage between two crests may be changed in the course of time to a wide expansive valley by the action of the rains, frosts, and other disintegrating agents, and the relative position of the strata forming its walls may seem to be entirely changed.
Fig. 6.
Suppose, for example, that the two upper layers of the strata rent apart by the upheaval of the mountain are limestone and sandstone, while the third is clay and the fourth again limestone (as in Figure 6). Clay is soft, and yields very readily to the action of rain. In such a valley the edges of the strata forming its walls are of course exposed, and the clay formation will be the first to give way under the action of external influences. Gradually the rains wear away its substance till it is completely hollowed out. By the disintegration of the bed beneath them, the lime and sandstone layers above lose their support and crumble down, and this process goes on, the clay constantly wearing away, and the lime and sand above consequently falling in, till the upper beds have receded to a great distance, the valley has opened to a wide expanse instead of being inclosed between two walls, and the lowest limestone bed now occupies the highest position on the mountain. Figure 7 represents one of the crests shown in Figure 6, after such a levelling process has changed its outline.
Fig. 7.
But the phenomena of eruptions in mountain-chains are far more difficult to trace than the effects thus gradually produced. Plutonic action has, indeed, played the most fantastic tricks with the crust of the earth, which seems as plastic in the grasp of the fiery power hidden within it as does clay in the hands of the sculptor.
We have seen that an equal vertical pressure from below produces a regular dome,—or that, if the dome be broken through, a granite crest is formed, with stratified materials resting against its slopes. But the pressure has often been oblique instead of vertical, and then the slope of the mountain is uneven, with a gradual ascent on one side and an abrupt wall on the other; or in some instances the pressure has been so lateral that the mountain is overturned and lies upon its side, and there are still other cases where one mountain has been tilted over and has fallen upon an adjoining one.
Sometimes, when beds have been torn asunder, one side of them has been forced up above the other; and there are even instances where one side of a mountain has been forced under the surface of the earth, while the other has remained above. Stratified beds of rock are even found which have been so completely capsized, that the layers, which were of course deposited horizontally, now stand on end, side by side, in vertical rows. I remember, after a lecture on some of these extravagances in mountain-formations, a friend said to me, not inaptly,—"One can hardly help thinking of these extraordinary contortions as a succession of frantic frolics: the mountains seem like a troop of rollicking boys, hunting one another in and out and up and down in a gigantic game of hide-and-seek."
The width of the arch of a mountain depends in a great degree on the thickness and flexibility of the beds of which it is composed. There is not only a great difference in the consistency of stratified material, but every variety in the thickness of the layers, from an inch, and even less, to those measuring from ten or twenty to one hundred feet and more in depth, without marked separation of the successive beds. This is accounted for by the frequent alternations of subsidence and upheaval; the continents having tilted sometimes in one direction, sometimes in another, so that in certain localities there has been much water and large deposits, while elsewhere the water was shallow and the deposits consequently less. Thin and flexible strata have been readily lifted into a sharp, abrupt arch with narrow base, while the thick and rigid beds have been forced up more slowly in a wider arch with broader base.