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.
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.
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.