gradual progress of this general operation is clearly discernible by the progress of vegetation'. Now, if an antiquity without any assignable limits is to be ascribed to our continents, such operations must long since have been every where terminated, and the surface of the slopes would be found entirely overspread with plants and verdure. But, remarks De Luc, the operation continues, and we observe its different stages, which depend on the original state of each individual part,. and on the nature of its strata 2. the actual form of all mountains, not only in Europe, but in all parts of the world, as may be judged from the views of various countries, which adorn the works of travellers; and it is so generally known as the character of mountains, that we find it to be impressed on the imagination of every landscape painter. What could have produced this form so universally, unless it were one general original state, acted on every where by the same causes?" See also Letter V. § 35. 1 These natural processes are minutely described and explained in Letter V. § 45.

2.66

Mouldering cliffs are excellent chronometers. Each has an accumulation of fragments constantly rising against the scarp. This in time will vanish, and instead of cliffs there will be hills, occupied by woods, by pasture, or by tillage." Rev. Joseph Townsend's Vindication of Moses, &c. Vol. i. p. 402.

"To the gradual softening of its slopes, is the valley of Campan indebted for being one of the most delightful retreats of pastoral life it was once a deep ravine......but the debris of the summits that command it have raised the bottom of those precipices; the waters have had a constant tendency to level the soil in their course, the process of crumbling down has been continued, rest has succeeded to long convulsions, and vegetation has covered those heaps of ruins now fitted for its reception. The valley of Campan may then be regarded as presenting by anticipation that appearance of repose described by De Luc, who has foreseen what mankind are to expect in regard to the perfectibility of our continents. Such will be all the valleys of the Pyrenees and the Alps, of Caucasus, of Atlas and the Andes." Journ. de Physique, tom. xli. (part ii.) p. 334. 1792.

Further-It has been rendered evident by De Luc, that the action of the sea' has a tendency to efface the original indentations of coasts, by diminishing the promontories, filling up creeks and bays, and reducing cliffs to gentle declivities. These simultaneous operations on the same coasts must have commenced as soon as the ocean retired into its present bed; they continue, and by the effects produced, compared with their measurable progress, necessarily constitute a direct chronometer. Accordingly all the lands formed by the sea upon its shores are every where distinct in their composition from those to which they are joined; they are horizontal from the point of junction to their actual extremity, and continue to increase. Now, from the known progress of these lands, compared with their total extent, it is manifest that the sea cannot have occupied its present bed during a very great number of centuries 2. Again, at the entrance of lakes, the progress of the accumulation of the

1 Professor JAMESON observes, that "De Luc, in his various works, has estimated this action with a correctness of observation and of reasoning, which is remarkable only, because it has not been adopted by all naturalists." Essay on the Theory of the Earth, translated from the French of CUVIER, p. 449. Fifth edit.

2 The first establishment of this particular chronometer in geology, is distinctly ascribed to De Luc by DOLOMIEU, in his Mémoire sur la Constitution Physique de l'Egypte: "I have already said, and I here repeat it; to the works of that philosopher it is, I am indebted for that luminous idea." See likewise Dictionnaire des Sciences Naturelles (article eau) by M. ALEX. BRONGNIART, where a similar acknowledgment is made.

It is remarked by the Rev. Joseph Townsend, that the great lakes exhibit accretions at the entrance of rivers into them, similar to deltas at the mouths of those which discharge themselves into the

sea.

"Thus," he says, " it is in the lake of Geneva, as particularly

sediments of rivers affords another striking chronometer of the same kind; and in every such place, the

noticed by De Luc. This forms an excellent chronometer. For had our continents existed myriads of ages before the time assigned by Moses to the deluge, the lake of Geneva had been long since filled with the sediments of its waters, and had become an extensive plain; because not an atom of this sediment either escapes out of the lake, or is deposited at any considerable distance from its entrance into it. The Rhone, at its departure from Geneva, having deposited all its impurities, is perfectly limpid, and although thirty feet deep, does not appear to have the depth of thirty inches." Character of Moses, &c. Vol. i. p. 398.

Speaking of the delta formed by the Rhone, in the lake of Geneva, Mr. J. A. de Luc observes, that "Mr. LYELL is to a certain point correct, when he asserts, in his Principles of Geology,' pp. 223, 224. that it was first necessary to ascertain the quantity of matter conveyed by that river into the lake, and to measure the extent of that part of it which is filled up, and its thickness; it is only by exact measurements that we can arrive at positive results. Although I have very frequently visited the spots, I could not state with precision how far our lake has been diminished by the new lands of the Rhone. The breadth and depth of the portion filled up are considerable. Its length may extend to three leagues, and its breadth to one league. At a league's distance from the mouth of the Rhone, the depth of the lake was from 90 to 104 fathoms, or about 600 feet. Much time is necessarily required for such a depth to be filled up by the matter carried down by a river. But although these calculations have not been made, it may nevertheless be justly concluded from the fact of so small a proportion of our lake having been filled up, that a great many ages have not elapsed since the flowing of the Rhone. Although the Rhone deposits its impurities in the lake, it still conveys sedimentary matter to the sea, collected from the tributary rivers of the Arve, the Ain, the Saone, the Isère, the Drome, the Durance, the Ardèche.

"I cannot refrain from observing that Mr. Lyell's arguments too often rest on gratuitous hypotheses. Thus he supposes that the Rhone may have flowed into the lake for thousands of years, without importing any sediment whatever; and states that such would have

dates at which possession was taken of new grounds, and embankments were opposed to the swells of the streams, are ascertained 1.

been the case, if the waters had first passed through a chain of upper lakes, and through the channels of many of its principal feeders. Of the existence in former times of such lakes, there is no proof. The bottom of the valley, indeed, is flat in the vicinity of Martigny, and between that town and Sion; but it was in all probability the waters of the ocean, at the time of its retreat, that levelled the bed of the valley, not the waters of the Rhone. Between Martigny and the lake there is no level ground, except what has been produced by the sediments of the Rhone on entering the lake. In a short memoir written a few years ago on the Alpine valleys which have their beds levelled, I came to the conclusion that such a state was produced by the waters of the ocean which levelled the valleys by the sedimentary matter they conveyed and deposited. The rivers may have afterwards contributed to produce the same effect, but not by filling up lakes. I do not understand the force of Mr. Lyell's argument, when he mentions the channels of many of the principal feeders of the Rhone. Its feeders are all the torrents that come down the lateral valleys or gorges; they flow very rapidly on account of the slope, and they have conveyed gravel, mud, &c. into that river, ever since the retreat of the sea."

In the following passage this chronometer is also admitted by the authors of the "Outlines of the Geology of England and Wales."

"The alluvial tract at the mouth of the Po has been ascertained by observation to have a regular rate of increase in a century; and the line where this tract begins, against what must have been the original coast, is capable of being determined; these data afford, it is obvious, sufficient grounds for calculating the length of time requisite to produce the whole of this alluvial tract." Outlines of the Geology of England and Wales, by CONYBEARE and PHILLIps. Introd. p. xxxii.

1 Speaking of the sedimentary matter which rivers bring down with them to the sea, the author strikingly observes: "This is the true clepsydra of ages, which takes its date from the last revolution; [namely, that which occasioned the interchange of land and sea,] the zero of time is there in a manner fixed by the unchanging level

C

The fifth Letter in the present collection likewise exhibits a masterly view of the several chronometers with which our continents supply us. Those which are formed by the progress of agriculture, and the various stages of cultivation, will excite peculiar interest, as affording one of the proofs of the small antiquity of the human race upon our present lands '.

of the ocean; and its degrees are marked by the accumulation of the deposits of rivers, as they were by the accumulation of sand in our ancient chronometrical instruments." Lettres sur l'Histoire de la Terre, &c. Vol. v. part ii. p. 497. 1779. See likewise the Fifth Letter of this Volume, § 29.

1

Adverting to this subject in his Travels in the North of Europe, §. 366, "The countries naturally fertile," says the author, were the first peopled; and there every trace of progress has long since been lost, as well in their culture, as in the number and manners of their inhabitants. But, as population increased, it extended itself of necessity over those less favourable soils, which, for a long time, had been thought wholly unfit for tillage, and capable only of affording shelter for game, or a meagre pasturage for cattle. It is true that, according to the most ancient monuments among all nations, agriculture, on these continents, is coeval with the human race itself; but at first it consisted only in the use of the plough for the purpose of raising corn, and in the dressing of the vine; by these, in good soils, the increase of population was promoted; but the art of fertilizing soils naturally sterile made very slow advances, and was taught only by experience and necessity. The barren lands, not occupied by forests, appear to have been, for the most part, covered with heath; and it is on these in particular that the progress of agriculture forms a true chronometer, by the various stages of fertilization observable on the soils brought into tillage, some much earlier than others; and by the consequent transformation of hamlets into villages, and of villages into towns, still retaining names denoting their small origin, as well as by the increase in the number of hamlets, in proportion as cultivation has extended itself over desert tracts; of this my present travels will contain additional examples." That our continents have successively emerged from the waters of 2*