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.
SECTION III.—CHANGES OF ELEVATION AND SUBSIDENCE.
The continents, if we except the more rugged and broken portions, rise from the sea with an almost imperceptible ascent; and even the mountains have a much gentler slope than we are apt to suppose, so that a section of the earth parallel to the equator would be almost a perfect circle. The slope of a mountain, from its base to its highest point, rarely forms with the horizon an angle of as much as twelve degrees. In the following figure ([57]), A represents the peak of Chimborazo, B of Teneriffe, C of Ætna, and D of Mount Loa, the principal volcano of the Sandwich Islands. The highest mountains would be represented on a twelve-inch globe by an altitude of less than the one-hundredth of an inch above the level of the sea. But the rising and sinking of these masses, though so small compared with the dimensions of the earth, are yet geological changes on the largest scale.
1. The Elevation of Mountains.—Mountains have formerly been covered with the waters of the ocean. This is evident, in the case of some mountains, from the existence of stratified rocks reaching to the summits. The stratification could have been produced only by deposition from water. It is, moreover, evident from the existence of marine fossils, distributed through these strata, so abundantly, that they cannot be accounted for on any other hypothesis than that the animals lived and died where the remains of them are now found. These strata must therefore have formed the bed of the sea while the fossils were accumulating.
There is no direct evidence that the granitic mountain peaks were ever submerged. But there is reason for believing that the sedimentary strata which now occupy the lower slopes were, at the time of their deposition, continuous,—the igneous rock having subsequently broken through them,—so that the waters of the ocean once rested on the whole area which the mountain now occupies!
If the ocean could ever have been above its present level sufficiently to have covered all the sedimentary rocks, we might assume that the height of mountains has not been changed. But the level of the ocean cannot be subject to much variation. The total amount of water on the globe is always the same. If the continents and mountains were all submerged at once, and the waters were expanded by the highest temperature consistent with the liquid form, there would not be a change of level of more than two hundred and fifty feet. We may assume, then, that the ocean level has always been essentially the same that it now is. We must therefore conclude that the sedimentary rocks, and the mountains of which they form a part, have been elevated to their present position from the bed of the sea.