is the only road by which we can ascend to the succession of physical events that were brought to pass on the globe by causes now no longer existing'. The first discernible effects, then, of these causes, were precipitations continued for a long time, of substances differing successively in their genera and species, and spread out in strata,—the greater part of them solid, and the rest soft,—over large tracts of the bottom of the liquid, and in a situation nearly horizontal2.

These strata, in the intervals of their formation, and until the end of these operations, were subjected to a series of catastrophes, in the course of which they were divided by fractures and dislocations into masses which became ridges of mountains, having undergone such angular movements that valleys were

Travels in France, &c., Vol. ii. p. 381. Journ. de Physique, tom. xl. pp. 180—197.

2 That the most ancient strata of our globe, now found strongly inclined, and sometimes even vertical, were yet formed in a horizontal position, was first ascertained by De Saussure, from his observations on the breccias or pudding-stones of the Valorsine. See the second volume of "Voyages dans les Alpes," §§ 689, 690. Strata, which originally were so soft as to enclose fragments of other stones, could have been formed of an equal thickness only in a horizontal position; their substance would have slidden to the bottom in one mass. Element. Treatise, § 142. 1809. The interposition of puddingstone, says our author, between the classes of homogeneous strata begins even among the primordial for M. de Saussure has shown instances of it, as well between granites and schists, as between these two first classes and the calcareous strata, and between the latter and the strata of sand-stone; besides many repetitions between different species of the same genera. Geol. Travels, Vol. i. § 76. 1810. Element. Treatise, § 320. The original horizontality and continuity of the strata of later formation, are evidently shown by the marine and other organic bodies contained in them.

:

necessarily produced in their intervals'. Our mountains then, in the opinion of the author, exist under

1

1 This, however, is not the only cause which has operated in the formation of valleys. There has been another cause, (as manifested by the phenomena,) which has been already adverted to, and upon which much light has of late been thrown by Von Buch, Mr. J. A. de Luc, jun., and other geologists. See note to Letter I. § 21. By some violent agency, the mountains have been, in many instances, cut asunder, whereby a passage has been opened for the waters, without the least angular motion of the strata. The transversal valleys in the great chains of mountains, observes Mr. J. A. de Luc, are not to be explained by the subsidence or elevation of the strata. "In respect to the valleys of denudation," (he further remarks) as they are termed by the English geologists, they have been excavated beneath the ancient sea which possessed a dissolving, or corroding power, and they must have required a long abode of its waters for their production. This was also the opinion of De Luc, when he ascribed so many phenomena to the operations of the ocean.'

66

The following general views respecting the formation of valleys have been recently communicated by Mr. J. A. de Luc, to the editor :

"When we take the word valley in its most general acceptation, as comprising not only the valleys which traverse the great chains of mountains, but also those which cross the hills and plains, we may distinguish eight modes of formation.

1. By subsidence, in such a manner as to occasion strata which have been formed horizontally to break and sink down on one side, and on the other to rise up and present their summits upwards. If two subsidences take place in opposite directions which meet towards the lowest extremity, the result is a basin or a valley.

2. By elevation, whereby the strata have been turned up, and great masses have been forced to assume a vertical position. In that case we can readily imagine that valleys must be formed on the two sides of those strata thus raised up.

3. By lateral pressures which have bent the strata, and have caused them to present their convexity either upwards or downwards.—These modes of formation are connected with the origin of chains of mountains, as yet unexplained.

their present form, only because portions of the strata, once a continuation of those which compose these

4. By violent fractures which have occasioned transversal breaches of more or less breadth, in ranges of continuous mountains, unattended by subsidence or elevation. Here we may place the valleys of disruption, spoken of by Professor Sedgwick, in the direction of the crevices and fissures produced by great elevating forces.

5. By the crosive and dissolving power of a liquid which has excavated the surface of a great extent of strata, and has thus hollowed out valleys, sometimes simple, sometimes ramified, called valleys of denudation. The same force has produced the diluvian gravel.

6. But what is the cause which has excavated the great chains of mountains, so as to form vast irregular basins, each comprising a great number of valleys of three different orders, which all open into each other, terminating in a general outlet which is that of the principal valley. Thus the basin of the valley of Aoste, which terminates at Ivrée, comprises twenty-five valleys of the second and third order; the valley of the Durance, which terminates higher than Avignon, comprises thirty-three. How have those systems of valleys been formed, which divide the whole chain of the Alps into irregular basins? This is the great problem which remains to be solved.

7. Another cause, which is only secondary, is that which has widened subsequently or simultaneously the gorges formed by transversal disruptions of the ranges of continuous mountains. We may conceive that at the moment of those disruptions which were produced in the bosom of the ocean, the violent motions in its waters thence resulting, occasioned them to bear away with them the debris, and to convey it to a distance. After the retreat of the sea, or of great bodies of water in general, the rain waters, the falling in, and crumbling down of large masses of all dimensions, the streams which form in those valleys, have all contributed to widen them, and have at one time excavated them more deeply, and at another time filled them up. [See Letter I. § 21. note.]

8. A great number of channels, more or less deep, have been excavated by rivers in the soil formed by transported matter, such as we observe them in the basin of Geneva, in which the Arve and the

eminences, have sunk around them.

From such sub

plains, and the

sidences have resulted valleys and cavities of lakes'; and the eminences already formed by anterior catastrophes, were left on our continents as mountains, by the removal of the ocean to its new bed.

Changes in the Nature of our Mineral Strata. Catastrophes which they have undergone. Causes of those two great Geological Phenomena indicated.

The successive changes in the nature of our mineral beds, and the disturbances which they have suffered, form two of the most important phenomena of geology. If the formation of the different kinds of strata is of necessity to be ascribed to precipitations successively differing in their nature, the changes in the pre

Rhone have hollowed out channels in the diluvian accumulations of pebbles, gravels, sands, clays, &c. to the depth of 100, 200, and 300 feet; but those channels are not, strictly speaking, valleys."

1 66 "That many of our mountains," says Mr. J. A. de Luc, "have been formed by the overthrow and subsidence of the strata, we cannot entertain a doubt. Of this fact we have several instances in the calcareous mountains which constitute the exterior range of the Alps near Geneva, and on the southern side of our lake. Our basin itself can have been formed only by a depression which has separated the chain of the Alps from that of Jura. The theory, however, of subsidence is applicable to some parts only of the Alps, and of the other great chains, particularly to the calcareous ranges." "Because some mountains," the same geologist remarks, "have been raised by subterraneous action, it has been inferred that the whole of our continents has also been raised. No conclusion can be more Letter to the Editor.

erroneous.

cipitations must as necessarily have been produced by corresponding changes in the liquid'. But, it would be only by the introduction of new ingredients that the changes in the liquid could be effected; and the mode in which these new ingredients were introduced, was by ascending through the crevices of the fractured strata from the interior parts of the globe2. These first and legitimate conclusions, deduced from existing monuments, serve, says the author, to explain a variety of phenomena that would otherwise be involved in obscurity.

The general cause of those catastrophes which determined the present form of the earth's surface, is particularly considered in the following Letters, and may be thus summarily stated:-The globe was originally composed of disunited particles, or pulvicules3,

1

Letter III. §§ 18, 19, 22. IV. §§ 17, 18. Journ. de Physique, Tom. xxxvii. (part ii.) p. 213. et seq. and p. 332. xl. p. 285. et seq. 2 Letter IV. §§ 16, 17. The following application of this theory to an interesting geological phenomenon occurs in the first volume of the Geol. Travels, § 185.-Adverting to the fragments of flint disseminated in the sand, near Berlin, as also in Westphalia, and the country of Bremen, "these flints," the author observes, “are of the kind belonging to chalk, though there is not the smallest appearance of chalk in the whole neighbourhood. In my Lettres sur l'Histoire de la Terre, &c., I have, as I believe, assigned it to its true cause: namely, that, after the primordial liquid had produced the strata of chalk, in which these flints were formed, new precipitations, occasioned by new ingredients proceeding from the interior parts of the globe, altered so much the state of that liquid, in certain places, that substances, before separated from it by precipitation, were again dissolved by it; a circumstance of which I have given other examples. Now when the chalk was dissolved, the flints, not being soluble, alone remained."

3 This word was first adopted by the author, who had found none