Sidor som bilder
PDF
ePub

of natural chronometers, that the antiquity of our continents is not greater than the Mosaic chronology from the deluge ascribes to them, thus afford us a powerful evidence of the inspiration of the sacred writer 1.

SECTION IV.

Two Periods in the History of the Earth.

The study of terrestrial phenomena, says De Luc, enables us to discern and distinguish two separate periods in the history of the earth 2; one prior, the other posterior, to the epoch at which our continents

1 In the sixth Letter of the present collection, and in the second part of the fifth volume of the "Lettres sur l'Histoire de la Terre," &c. the coincidences in question are very fully considered.

Letter V. § 11. Journ. de Physique, tom. xxxix. (Part ii.) p. 333. 1791. Geol. Travels in France, Vol. ii. p. 375. The Saturnian, or antediluvian, and the Jovian, or post-diluvian, periods, established by M. Alex. Brongniart, in his classical work, entitled, "Tableau des Terrains qui composent l'écorce du globe," (1829), bear the closest analogy to the two great periods of which an account is given in the text; the distinction indeed was evidently suggested to that able naturalist by the works of our author. The Jovian period, according to M. Brongniart, "includes all terrestrial phenomena that are beyond doubt posterior to the structure and form of our continents, i. e. every thing that may have been operated at the earth's surface, without the necessity of having recourse for its explanation to powerful causes, the action of which is no where observable, dating from historical times...... The Saturnian period comprehends all the geological phenomena, the existence of which cannot be accounted for without calling in the aid of forces which are no longer in action." The most important of those phenomena are enumerated in pp. 63, 64. of his work.

began to exist. To such a distinction we are led by the discovery that the part of the globe which is at present inhabited by man, was once the bed of the sea; for the first period must evidently have been that of the continuance of the globe in this first state; and the second must have begun at the epoch when the bed of the ancient sea became dry land. This last period the author calls the modern history of the earth,—in which we have a fixed chronology, and which has already been here brought under consideration the former he designates by the appellation of its ancient history, which comprises that series of geological events in which we discern only a succession of subordinate periods, without any determination of time 1.

It is to the want of discriminating between these two periods, and between the phenomena separately assignable to each, that the author ascribes the errors of the earliest geologists. They had supposed that our continents, since they have existed as such, have been the theatre of certain operations, which are now demonstrated to have taken place on the bed of the sea; and as this mistake rendered it impossible to attribute these operations to any other causes than those known to be at present acting on the surface of the globe, it has hence been found necessary to allow to the action of these causes a time without any determinate limit; because it cannot be shown that, within a known period, they have produced any similar effect. Of this De Luc adduces as an ex

1 Letter V. § 11. Travels in France, &c. Vol. ii. p. 375. Lettres sur l'Histoire de la Terre, &c., Vol. v., Part ii. p. 488. ? Travels in France, Vol. ii. p. 376.

ample, "the supposed excavation of valleys by the action of running waters; an operation which would require a time absolutely illimitable, since no actual progress in it can possibly be pointed out1." He has, however, on the contrary, made it appear that streams, far from having excavated their beds, have in a variety of places raised them to a higher level than they originally possessed; and has proved that the cavities of the pools and lakes which still exist along the courses of so many rivers, must have been filled up, if the valleys in which those rivers flow had actually been hollowed out by them 2.

1 Travels in France, Vol. ii. p. 376. See also De Luc's "Geological Travels," Vol. iii. p. 520-528. "Table of Geological facts." In that volume the author combats the error of Prof. Playfair, "in his not allowing for any other agents in the production of valleys, than those which, since the continents have existed, have exercised their action in them." p. 372, et seq.

Professor SEDGWICK is of opinion that the deep gorges and river channels in the high regions of Auvergne, were excavated solely by the attrition of the rivers which still flow through them. Dolomieu, in 1798, had combated this opinion in the Journal des Mines, observing that the valleys of Auvergne could have been hollowed out only by an extraordinary force which is now no longer in action; for it is not by causes now in operation that nature could have produced such excavations. The rills of water which run in the valleys of those districts could not have excavated the mass of a granitic rock, frequently extremely hard, to the depth of six hundred feet, and including a breadth sometimes of half a league, leaving lateral embankments nearly similar to the walls of a rampart.

2

Travels in France, &c., Vol. ii. §§ 498, 562.

"Dr. Hutton's assertion," observes Mr. J. A. de Luc, "that on our continents there is no spot on which a river may not formerly have run,' is without any foundation; there are extensive tracts of country in Europe which have never been watered by streams. In England, for instance, no river has at any period ever flowed over Hampstead,

But by an attentive study of our continents, it is possible, according to the author, to discover the effects of causes which have been in action on the bed of the sea, antecedently to its retreat, but which have since ceased to operate'. By the same enquiry

Highgate, Shooter's Hill, Black Heath, Epping Forest, and yet those hills are covered with water-worn flint-gravel. The channels of rivers having been produced by antecedent catastrophes, were already prepared to receive and collect their waters, and since the retreat of the sea from our continents, the rivers have deviated very little from their original beds. Most of the pebbles occurring so abundantly in the beds of rivers, have not been brought down by them; they were found in the country, and are identical with those still scattered over the neighbouring hills, where no river has ever flowed. But Mr. Lyell asserts, p. 70, that De Luc generally reasoned against Dr. Hutton, as if the latter had said, that the existing rivers flowing at their present levels, had caused the excavation of valleys.' If Mr. Lyell had quoted De Luc's own words, the reader would have seen that my uncle made no such supposition as Mr. Lyell ascribes to him."

1 The author states in his Elementary Treatise, that in the course of their geological career, his brother and himself came to a first conclusion, which thenceforth was their guide, and which he always considered as the true introduction into the field of geology. "We saw," " he says, "that an essential distinction was to be made among the various phenomena which the surface of the earth exhibits, with respect to their causes; determining in regard to each of them whether the causes which have produced it are still in action, or have, at some epoch, ceased to act. If this discrimination be possible, it evidently becomes a first guide in the research of causes, which will prevent many errors. Now, when we had fully convinced ourselves that this distinction was pointed out by the phenomena themselves, we clearly saw, on studying the theories of the earth then known, that the principal source of the errors, which had been detected in them by subsequent observation, was the confounding of the periods in which certain effects had been produced. For, by attributing to causes which were seen in action, such effects as they were inca

we may ascertain the cause which, in particular, produced the retreat of the waters from the surface of

pable of ever producing, an impenetrable veil was thrown over past causes; since these can be discovered only by their real effects, more surely to be ascertained when separated from those produced by causes, which are still operating, and producing such effects, as may be discerned to belong to them. Continuing our observations with this object in view, we came at length to a conviction that the production of the mass of our continents, in regard both to their composition and general form, as well as their existence above the level of the sea, should be ascribed to causes no longer in action on our globe; and that the whole of the effects of existing causes had been limited to the modification of this original state. Insomuch that the first study requisite to the investigation of the past history of the earth, was that of the action of causes now in operation, that thus, by their being every where determined, our continents might be traced back to their original state." Pp. 36, 37.

Alex. Brongniart, as has already been stated, and Cuvier [See Vth sect. of this Introduction] concur in recognising the important distinction here made between the causes which have ceased to act, and those which are still operating. This distinction is denied, upon insufficient grounds, by Mr. Lyell, in his "Principles of Geology." Valuable remarks on this subject, occur in a series of Letters by the Rev. W. Conybeare, inserted in the New Philosophical Magazine, 1830, and continued in the present year.

"It would appear," says Mr. J. A. de Luc, jun., " that Mr. Lyell would bring back geologists to the opinion of Hutton and Playfair, that existing causes are the same with those that have operated in ancient times-an assumption which is inadmissible--and that such causes are sufficient to explain all the phenomena. His theory is the same with that of Prof. BERTRAND of Geneva, respecting successive renewals of continents, and successive deluges. It is somewhat surprising that such systems should be revived.-One would imagine that Mr. L. is desirous of indirectly combating De Luc, who showed that there had been a commencement to terrestrial phenomena, and that the commencement was the creation of light."―[See the sequel of the present section.]

Some sound reasoning in refutation of Mr. Lyell's doctrine, that

« FöregåendeFortsätt »