Imágenes de página

the rivers to new lines and points of debouchure, is equally untenable.

"On the other band, if a current charged with sediment vary its course—a circumstance which must happen to all of them in the lapse of ages—the accumulation of transported matter will at once cease in one region, and commence in another.

"Although the causes which occasion the transferrence of the places of sedimentary depositions are continually in action in every region, yet they are particularly influential where subterranean movements alter from time to time the levels of land; and their effect must\e very great during the successive elevations and depressions which must be supposed to accompany the rise of a great continent from the deep. A trifling change of level may sometimes throw a current into a new direction, or alter the course of a considerable river. Some tracts will be submerged and laid dry by subterranean movements; in one place a shoal will be formed, whereby the waters will drift matter over spaces where they once threw down their burden, and new cavities will elsewhere be produced both marine and lacustrine which will intercept the waters bearing sediment, and thereby stop the supply once carried to some distant basin.

"Without entering into more detailed explanations, the reader will perceive that according to the laws now governing the aqueous and igneous causes, distinct deposits must at different periods be thrown down on various parts of the earth's surface, and that in the course of ages the same area may be again and again the receptacle of such dissimilar sets of strata."—Principles of Geology, vol. ii. p. 212.

But this does not account any more than the other for the diffusion of the materials of any one of the strata over the whole bed of the ocean; it only indicates a mode in which the points where rivers enter the sea may be changed from time to time so as to produce a change of the areas at the margin of the ocean, where the sediment borne down by them shall be deposited. It leaves all the other parts of the bottom of the sea as unprovided as they were before, with materials for the formation of new strata. In order to explain their deposition over the whole bed of the ocean by such a cause, it would be necessary to suppose, not only that the continents from which the materials of the strata were derived, but that small divisions of the bottom of the sea itself, also, were successively elevated, so that the river should in succession enter it at as many points as would be requisite, in order to the deposition of a stratum over its whole area. That, however, cannot have happened; inasmuch as the elevation and depression of the surface in detached parts that must then have taken place at the formation of each layer of the system, would have broken their whole mass into fragments, and reduced them to a promiscuous heap of ruins. But they have suffered no such violence. The New York or Silurian groups which underlie the whole country from the Alleghanies far into the Canadas, and from Vermont beyond the Mississippi, are but slightly dislocated. Throughout a large part of their immense area they lie at a dead level, or moderate inclination, and have never been seriously disturbed in any of their members since their deposition. It is clear then beyond debate, that their materials were never transported to their several places by the action of rivers. The supposition is indeed so palpably at war with the laws that govern their agency, and so absurd and enormous an extravagance, as to render it surprising that considerate persons should have ever entertained it, and made it the basis of an argument for the immeasurable age of the world.

And finally, in addition to all these evidences of the error of their theory, the distribution of the elements, silex, alumine, and lime, of which the formations chiefly consist, into separate strata, is an equally decisive proof that they cannot have been drawn from such pre-existing continents, nor been borne to their several places and arranged in their positions by the agency of streams, rivers, and currents. The detritus that is wafted down by rivers to the sea, is not separated, on its deposition, according to the species of which it consists, and its different ingredients thrown down on different areas. Instead, they fall together and form a promiscuous mass. The only separation that takes place, is of the heavier from the lighter grains and particles—gravel falling first, sand next, then comminuted mud, and last, light vegetable particles; and as their fall takes place by the force of gravity, the points at which they severally descend are determined by their weight, not by the material of which they consist.

The strata, however, were not formed in that manner, but their great elements were distributed into separate layers; sandstone, sand, and gravel, of which silex is the chief ingredient, being arranged by themselves; slate, marl, and clay, which owe their principal character to alumine, forming a different set of layers; and limestone and chalk, of which lime is the great ingredient, constituting beds and masses by themselves. The sand, gravel, and pebbles, moreover, that enter into the composition of many of their layers, instead of being sorted according to size and weight so that they regularly decrease in dimensions in one direction, and increase in the other, as in the deltas of rivers, are distributed indiscriminately throughout the spaces—sometimes of vast extent—which they occupy. Thus the sandstones of the New York system stretching, there is reason to believe, from Vermont to the Rocky Mountains, and from the Appalachians far into the Canadas, do not vary in the coarseness of their grains and pebbles in any ratio to their distance from their eastern, western, southern, or northern edges, or from any interior points in the area over which they are spread. Nor do those of the carboniferous system, which extend from the Alleghanies through the western parts of Pennsylvania and Virginia, the southern part of Ohio, and a large share of Kentucky, Indiana, and Illinois. Their variations, if they vary, are obviously from chemical, not from mechanical or geographical reasons.

These facts form, therefore, the most unanswerable demonstration, in the first place, that the materials of the strata were not derived, as the theory represents, from disintegrated continents of granite—as they could not then have been assorted as they are, and their several elements arranged in layers by themselves; and in the next place, that the distribution throughout their whole extent, of the sand and gravel that enter into the composition of sandstones and other strata, were not transported in their present form from a distance to their places, by the action of streams and currents.

Such are the proofs of the error of this extraordinary theory, which refers the materials of the stratified rocks to anterior continents and islands of granite. There is not a solitary step in the process which it contemplates at which it is not confuted by a palpable contradiction to the laws of the physical world, and the principles of geology. The supposition with which it commences of the creation of the earth in a

[ocr errors]
« AnteriorContinuar »