Instances enough have now been given to show how extensively the system admits of change. They are sufficient to justify us in searching for indications of great revolutions in past times, even where no such indications have as yet been discovered. They will serve as a key to many otherwise inexplicable phenomena, In order to the interpretation of such phenomena readily, we must cease to look upon these as exceptional cases, and regard them not only as facts, but as facts of frequent occurrence.
From the examples which have now been given, as well as from speculations upon the cause of these changes, it seems highly probable that all the surface of the solid portion of the earth, whether land or the bed of the sea, is undergoing changes of level. It may be so gradual that in the life of an individual it would be imperceptible, even where the best means of detecting it exist. These means are generally the works of man, and they are themselves so liable to change, that it would be scarcely possible to detect variations of level, which amount to but a few inches in a century.
If we admit that the relations of land and water have always been variable, it is impossible to arrive at any certain conclusion as to the amount, position or form, of the dry land at any former period. We may determine, with some degree of certainty, what portions of the present continents were submerged at particular epochs. Thus, we may infer that most of this country was submerged during the silurian period, from the great extent of the Silurian rocks; and, from the limited extent of the chalk formation in this country, we know that during the cretaceous period most of the continent was above the surface of the sea. But we have absolutely no data for determining what portions of the bed of the sea were at any time dry land.
It is supposable that the land has been principally confined to the equatorial regions at one period, and to the polar at another. At still a different period the land may have existed as islands scattered through a general ocean. These relations may, therefore, be assumed to have existed, if there are geological phenomena which best accord with such relations.
SECTION IV.—CHANGES ON THE SURFACE OF THE EARTH.
1. The principal changes of this class consist in the wearing down and removing immense quantities of the surface rock. The form in which the igneous rocks, of which the entire crust of the earth was originally composed, now appear, furnishes no assistance in judging of the amount of denudation which they have suffered. We can judge only from the amount of rock for which they have furnished the materials, and these are the whole sedimentary series which exist both as dry land and as the bed of the sea.
2. The sedimentary rocks have also been subject to great denudation; and we often have, in what is left, some indications of how much has been removed. One of these indications consists in the now level surface of those portions of country in which large faults exist. By the excavations for coal, in England, faults have been discovered of five or six hundred feet. At the time that they were formed, the surface must have presented precipitous escarpments (as represented by the dotted lines in [Fig. 50]) of a height equal to the dislocation; but the whole is now reduced to a general level (z z), denuding causes having removed the elevated portions.
The extent of valleys will often give some idea of the amount of denudation to which a region has been subjected. In the north-west of Scotland there is a succession of hills of about three thousand feet in elevation, consisting, for the upper two thousand feet, of horizontal strata of old red sandstone. ([Fig. 66.] We cannot conceive that these mountain masses were deposited in their present isolated form. The whole intervening spaces must have been filled with strata continuous with those by which the elevations are formed.[A]
[A] “I entertain little doubt that when this loftier portion of Scotland, including the entire Highlands, first presented its broad back over the waves, the upper surface consisted exclusively, from one extremity to the other, of a continuous tract of old red sandstone; though, ere the land finally emerged, the ocean currents of ages had swept it away, all except in the lower and last raised borders, and in detached localities where it still remains, as in the pyramidal hills of Western Rosshire, to show the amazing depth to which it had once overlaid the inferior rocks.”—Miller, Old Red Sandstone, p. 22.