-not many ages have elapsed since the continental parts of our globe were abandoned by the waters of the

ocean.

66

When," he says, we have studied the continents with attention in all their parts, from their centres to their borders; from their greatest elevations to their lowest depths; when we have seen the regularity of the strata composing the whole of their observable mass, and the quantity of the marine bodies contained in those strata, it is impossible not to acknowledge that we are inhabiting an ancient bed of the sea'."

But that a comparatively short period of time has elapsed since the retreat of the ocean from our pre

cannot be any other than "aqueous vapour.” He supposed "that the waters which circulate in galleries above those wherein lavas are formed, commonly find free issues; but when, in consequence either of some obstacle in their course, or of some increase in their volume, they rise in these channels, they then meet with crevices through which they penetrate down to the furnaces where the lavas are prepared. Among physical causes, there is no one that possesses a power equal to that which may be exercised by the aqueous vapour produced in these subterranean furnaces, while it subsists with the same degree of density; as is the case so long as it only extends itself along the deep galleries in which the matter of the lavas is formed by a kind of combustion; but when at last this vapour enters higher passages, where it no longer finds an equal degree of heat, it is decomposed, and thus loses its force." In that earliest of the author's works, he had expressed the opinion, that the basalts scattered on our continents are the product of lavas which burst forth at the bottom of the ancient In Vol. IV. pp. 258-260. 472; and Vol. V. pp. 362–369. will be found the proofs that the extinct volcanos on both banks of the Rhine have burnt beneath the waters of the ocean.

sea.

1

1 Travels in England, vol. ii. p. 927. Letter V. § 6. Lettres

sur l'Histoire de la Terre, &c. Vol. v. p. 456. et seq.

sent tracts of lands, is a position which by some naturalists has been contested. Several slow causes of a gradual retreat from its former bed had been imagined; but by continued observations of terrestrial phenomena, the author was enabled to demonstrate, in the above mentioned work, that all such slow causes were purely imaginary.

Geological Chronometers.

The following is the nature of the proofs by which, on the other hand, he was led, in the study of the earth, to infer the recent origin of our continents. If, he says, they are of small antiquity, all the classes of phenomena which indicate a succession, and which appear to date their origin from the existence of the new lands, are to be traced back to the same period; with as much exactness, at least, in respect both to the data and calculations, as regard to the nature of the objects can require'. Now, it is from such different classes of phenomena, that he has succeeded in establishing, with a precision of which the subject could scarcely be deemed susceptible, a well defined physical chronology of the earth, by means of what he has termed natural chronometers. It is from the documents of nature, and not from those of history, that he has deduced the chronology of our continents, and that of the human race.

From the period of the retreat of the sea from our lands, they have been subjected to a series of opera

1 Lettres sur l'Histoire de la Terre, &c. Vol. i. p. 11.

tions which are of such a nature as to leave monuments of their progress from certain points at which it is evident that they must have commenced; their action still continues, and is carried on before our eyes'. "Every natural object upon our continents," he says, "is a chronometer; and that, from the same general cause, viz. that every object which was liable to undergo a change, by reason of the state in which it then was, and in consequence of the action of physical or mechanical causes, began at that time to suffer such a change. We are thus supplied with an infallible guide in the history of the earth, through which we are conducted by unquestionable monuments. Wheresoever, by any perceptible cause, some part of our continents has suffered a change since the time at which they emerged from the sea, we can discern what was, its particular state at that period; and whilst we are enabled to ascertain the total change, we find portions produced within known times 2"

The operations of those causes have been ascertained by the enlightened author of the present work with singular ingenuity and success. All their products and effects are capable of measurement; they are absolutely distinct and independent of each other, having nothing in common but the point of their commencement; yet, however different in their nature, they all lead to the same conclusion:-the impossibility of carrying back the origin of our con

1 Geol. Travels. Vol. i. § 99.

2

Abrégé de principes et de faits concernans la Cosmogonie et la Géologie, p. 92. Brunswic, 1803.

tinents to a period more remote than that which the Mosaic chronology has assigned to the deluge.

Various classes of these chronometers are minutely described in the author's "Lettres sur l'Histoire de la Terre et de l'Homme," in the Journal de Physique', and in his Travels, as resulting from numerous phenomena;-such as the collection of vegetable earth produced by the decomposition of plants on uncultivated soils the natural history of peat mosses-the extension of snow and ice on high mountains-the accumulations of fallen materials under the abrupt sides of mountains, and at the foot of steep coaststhe reduction of cliffs into grassy slopes-alluvial lands formed by rivers along their course-maritime new lands, &c. We are here presented with chronometers, all possessing the same conditions; and consisting of effects resulting from known causes, which can have been in operation only since the continents have existed, and of which the progress within known times is indicated by indisputable monuments. More

1 See the 26th, 27th, and 28th Letters addressed to M. de la Métherie, tom xli. (part ii.) 1792.

"De Luc, to whom geology is indebted for more numerous facts than have ever been presented to the world, before he brought them forward to our view, is the first philosopher who thought of looking to this habitable world itself for the record of its birth. He has examined numerous physical chronometers, and demonstrated that these agree with revelation." Rev. Joseph Townsend's "Character of Moses Established for Veracity as an Historian.” Vol. i.

p. 398. By "habitable world" is meant our present continents. It may here be proper to notice, that when De Luc speaks of their birth, he adverts to the period when they were abandoned by the sea.

3 The following geological chronometer is noticed by Mr. R. C. TAYLOR in his interesting geological dissertation on the eastern part of Norfolk. Speaking of the Roman Causeway, supposed to have

over, the completion of several of these effects has been pointed out by the author in a variety of places'.

For the purpose of here further manifesting the nature of geological chronometers, as traced by De Luc, some of those just mentioned may be selected for remark.

Gravity, and the action of the rains, and other atmospherical causes, have a continual tendency to crumble down and reduce the abrupt sections of mountains, as well in valleys, as towards the plains. The detached fragments are accumulated at their feet, and "rise against them with that kind of slope, which, in fortification, is called a talus, being formed by materials rolling down over each other from a certain point;" and preventing the farther demolition of the parts which they thus have covered. The

been made by the Emperor Severus, which extended from Denver to Peterborough, across the Fens of Cambridgeshire, he "It says: was composed of gravel three feet deep and sixty feet broad, but is now covered with moor or peat from three to five feet in thickness. Evidence is thus produced of an increased elevation of surface, and the gradual formation of solid land, either by the deposition of oozy sediment, or by the growth and decay of vegetable substances; and data are supplied, as in the case of the ancient anchors, in the Garienis, for measuring the extent and duration of that process."

1 Journ. de Physique, tom. xli. (part ii.) pp. 229–239. 1792. 2 Geol. Travels. I. § 103. "No man," he there remarks, can travel round mountains, or along their valleys at any height, without having examples of these effects before his eyes. In great mountains, particularly, we see abrupt sections, one above another, under each of which we may observe these taluses, composed of the fallen materials, and forming amphitheatres of the greatest beauty, from the vegetation which more or less overspreads them, and from the variety of its products. This aspect is absolutely general; it is