Imágenes de página

owe their existence, those facts themselves do not demonstrate that deduction. In order to sustain it, they must prove that the materials of the strata were drawn from the sources to which they refer them; that they were borne to their respective places, arranged in their combinations, and subjected to the modifications which they have undergone, by the forces to which they ascribe those processes. If they cannot verify these hypotheses, if they are inconsistent with the facts of the science and the laws themselves of matter, then their deduction from them of the vast age of the world falls to the ground. On the other hand, the claims of that inference to be regarded as a scientific deduction will be confuted, if we simply show that the postulates from which it is drawn are merely supposititious, not demonstrated. If, in addition to that, we prove also that they are altogether irreconcilable with the facts and principles of the science, and the laws of nature, and infinitely self-contradictory, we shall furnish all the evidence that can be required to overthrow their theory; and that we propose to do.

Of the two great postulates on which they mainly found their deduction of the great age of the world, that which relates to the geological agents is, as stated

in a former chapter, that the forces by which the · strata of the earth were originally formed and subse

quently modified, were those of chemistry, fire, and

water, which are now acting on the globe, and producing somewhat similar effects; and that the energy with which they are now exerting their powers, and the scale on which they are giving birth to changes in the earth's surface, are to be taken as the measure of their intensity, and the rapidity with which they wrought their several effects in the formation of the strata. There is, indeed, some diversity of opinion among them in respect to this branch of the hypothesis. Thus Sir H. T. De la Beche says:

"The two prevailing theories of the present time are—1st, That which attributes all geological phenomena to such effects [operations] of existing causes as we now witness ; and 2d, That which considers them referable to a series of catastrophes, or sudden revolutions. The difference in the two theories is not in reality very great; the question being merely one of intensity of forces, so that probably by uniting the two we should approximate nearer to the truth.”—Manual of Geology, p. 32.

He accordingly, and all others who regard the formation of the strata as having occupied immense periods, hold that though at some few stages—as in the elevation of mountains, the dislocation of the strata, and their subsequent denudation - volcanic fires and the waters of the ocean must have acted with far greater energy than ordinarily; yet that, in the main, the rate at which they are now giving birth

to their several effects is to be taken as the measure of their past agency. The mode in which this theory is advanced by them was exemplified in a former chapter, by a variety of passages from the leading writers. We add a few others :

"It is only by carefully considering the combined action of all the causes of change now in operation, whether in the animate or inanimate world, that we can hope to explain such complicated appearances as are exhibited in the general arrangement of mineral masses.”Lyell's Principles, vol. ii., p. 210.

“The geologist must on no account think it out of the bounds of his legitimate province to examine with care and interest into the history of the processes now performed in the ocean and on land; for it is only by discrimination and generalization of these that we can hope to draw satisfactory inferences concerning the force and direction of the agencies formerly exerted in earlier oceans, and on earlier continents." -Phillips's Guide, p. 102.

"It is presumed that the reader will ... be convinced that the forces formerly employed to remodel the crust of the earth, were the same in kind and energy as those now acting ; or at least he will perceive that the opposite hypothesis is very questionable.”—Lyell's Principles, pref. xi.

"Moving water is the only agent known to us capable of carrying away the great collective mass of rock”—that has been swept from the mountains and hills. “In order, therefore, to form a just conception of the time and conditions required to produce the effects observed, we should carefully examine the latter, and estimate the transporting powers of those waters which now exist among the mountains themselves, and which transport detrital matter from the central parts outwards."— Sir H. T. De la Beche's Theoretical Geology, p 147.

“The immense period requisite to wear away such a mass of rock as this theory supposes to have once occupied the whole valley of the Connecticut, will seem to most minds the strongest objection to its adoption ; I mean, supposing it to have been effected by such causes as are operating at present. But this is not a solitary example, in whichi geological phenomena indicate the operation of existing causes through periods of duration inconceivably long. We may in this case, indeed, suppose the occurrence of other agencies in the earlier periods of our globe. Still even with this aid the work must have been immensely protracted. And why should we hesitate to admit the existence of our globe through periods as long as geological researches require ?" -Hitchcock's Geology of Massachusetts, p. 339.

These views are advanced by a crowd of other writers. There is no element of their speculations in which they more generally agree, than that the causes to which the strata owe their origin and modifications, were those now in activity on the

globe, and that they produced their effects by agencies in the main of only their present energy.

Their other great postulate is, that the stratified · portions of the crust of the earth were formed mainly from the detritus of rocky continents and islands, and borne down to the ocean by rivers, or beat off by waves from the shores, and distributed over the bottom of the sea by tides and currents. Thus Dr. Buckland :-

“Beneath the whole series of stratified rocks that appear on the surface of the globe, there probably exists a foundation of unstratified crystalline rocks, bearing an irregular surface, from the detritus of which the materials of stratified rocks have in great measure been derived, either directly by the accumulation of the ingredients of disintegrated granitic rocks, or indirectly by the repeated destruction of different classes of stratified rocks, the materials of which had, by prior operations, been derived from unstratified formations, amounting to a thickness of many miles. This is indeed but a small depth in comparison with the diameter of the globe ; but small as it is, it affords certain evidence of a long series of changes and revolutions, affecting not only the mineral condition of the nascent surface of the earth, but attended also by important alterations in animal and vegetable life.

“The detritus of the first dry lands being drifted into the sea and there spread out into extensive beds of mud and sand and gravel, would for ever have remained beneath

« AnteriorContinuar »