the strata by which they were once covered: this constitutes what is called a denudation. Instances of such denudations are of frequent occurrence.

Mountains, except those formed by volcanos, are seldom isolated masses rising from a plain, but they form groups, or are ranged together in a certain direction, and compose long and lofty ridges, denominated mountain chains. Lower ranges of mountains, running in the same direction as the principal range, and separated by valleys of greater or less width, may be observed accompanying almost all very lofty mountain chains. This fact appears to indicate the operation of a powerful powerful elevating force, acting in one direction along a certain line, and decreasing in intensity as the distance from each side of this line increases; but this action does not appear to extend with equal force on both sides of the line, for the smaller chains parallel to the great chain are seldom so numerous on one side of it as on the other. principal mountain chain, if very large, has its sides furrowed by small lateral valleys, and has not been unaptly compared to a back-bone or spine, with diverging ribs.

The

The shape of many countries and islands is evidently determined by the direction of the grand mountain chains that run through them.

The principal mountains in Europe and Asia, when viewed on a large scale, may be considered as forming a mountain chain composed of numerous mountain groups, and extending in an easterly direction from Cape Finisterre in Spain, to the most eastern extremity of Asia. Various parts of this chain receive different denominations in the different countries through which they pass. The Pyrenees, the Alps, Mount Taurus, Mount Caucasus, the Altaic, and the Himmaleh mountains, and the Yablonnoy mountains of Tartary, which extend nearly to Behring's Straits, may be regarded as forming together one immense mountain chain, and dividing

the northern from the southern dry land, both in Europe and Asia.

In North and South America one unbroken chain of mountains runs in a northerly and southerly direction for eight thousand miles, near the western side of that vast continent, and, with some minor diverging chains, has evidently determined the general outline of both countries.

A remarkable similarity occurs in the position of the escarpments or steep sides of mountains in the same mountain range. Various opinions have been formed respecting the law which the position of the escarpments appears to follow, but I believe the rule I submitted to the attention of geologists in the first edition of this work, will be found to approximate to the truth.

This

Mountain chains or ranges present the steepest declivities on the sides nearest to the sea. is remarkably the case in the long chain of the Alleghany mountains on the eastern side of America, which are steep towards the Atlantic. On the contrary, the Stony Mountains, which run near the northwest coast, and the Andes, near the southern Pacific Ocean, are steepest on their western side. In ranges of mountains that form the boundaries of lakes or of extensive vales, through which large rivers flow, the mountains nearest to the rivers have the steepest declivities. The largest rivers have their origin from the sides of mountains which are most inclined to the horizon, and most remote from the sea.

The beds or strata of very lofty mountains are generally much inclined, and are sometimes nearly vertical. Among these highly inclined beds, we not unfrequently observe beds of limestone containing marine shells, which must have been originally deposited at the bottom of the ocean. In some instances we meet with vertical strata, containing rounded pebbles and water-worn fragments of other rocks; these must also have been originally deposited on a surface nearly horizontal: we are therefore certain, that the

present vertical position of these strata is not their original one, and we hence also learn, that all the strata associated with them in the same mountain, and having the same inclination, were raised together. We have further proof that, before the epoch when this great revolution was effected, all these beds were covered by the seas then existing, and it was under the ocean that the change of position took place.

No person who reflects on the appearances presented in a mountainous district can believe that the broken and elevated beds, the peaked summits, the impending cliffs, and the immense fragments of rock scattered in the valleys and adjacent countries, were originally created and placed as we now observe them.

The traveller who, in crossing an extended desert, should meet with the remains of some unknown temple, could not for a moment doubt that the broken and prostrate columns, the mutilated arches, the scattered capitals and inscriptions, had been removed by some devastating cause from their original position; nor is the proof less certain, that the rocky pavement of our globe has been broken, and its parts, which were once united, widely separated from each other. Some of the phenomena we observe in mountains were produced by the disturbing force which first elevated them; others have been subsequently effected either by vast inundations, or by torrents that have torn away considerable portions of the softer beds, or by the more gradual decomposition and disintegration produced by atmospheric influence; by the latter cause, the lofty and exposed peaks and escarpments of rocks are constantly wearing down.

During the two summers I passed in the Alps, I was much struck with the circumstance, that all the great openings or passages over these mountains, called Cols, were made by excavations in beds of soft slate; and the fact I think admits of an easy

explanation, but I do not know that it has been before remarked by geologists.

If we suppose a portion of the Alps to be represented, Plate II. fig. 2., the dotted lines above the present surface will mark the supposed original prolongation of the different beds, at the period when they were raised. As the ocean, from whence these beds were raised, must have been agitated with inconceivable violence, the retiring waters would scoop out deep excavations in the softer beds of schist, and also tear off many of the vertical plates of the hardest rocks, and form the rudiments of these pyramidal peaks and aiguilles, which rise like the spires of a Gothic cathedral. Mountain torrents, caused by thunder-storms or the sudden melting of alpine snow, may have subsequently torn away large portions both of the harder and softer beds: the disintegration of the granitic aiguilles which are exposed to the influence of atmospheric agency is daily taking place, and their ruins are every day falling on the surface of the glaciers, and are carried down into the valleys: their peculiar forms are derived from their laminated structure, which disposes them to split in a vertical direction."

It is important to observe, that different groups and ranges of mountains have been elevated at different and remote epochs, and the birth of different parts of the same continent was not coeval: the more lofty parts constituted separate islands, before the whole surface emerged from the ocean. Satisfactory evidence of this will be adduced in a subsequent part of this work: it is sufficient to the present purpose to state, that the ocean has covered all that is now dry land, but not at the same epoch.

* Plate II. fig. 2. represents the general position of the beds near the Col de Balme and Mont Blanc; a a a, alternating beds of sandstone and limestone; bb, elevated beds of puddingstone, containing rounded stones and fragments of the lower rocks; cc, soft slate, in which a passage or col is formed; ddd, vertical granitic beds rising in pyramidal forms, called Aiguilles or Needles.

78

CHAP. V.

ON ROCKS DENOMINATED PRIMARY, AND THE CHANGES TO WHICH THEY HAVE BEEN SUBJECTED.

The Origin of Rocks called Primary believed by many Geologists to be igneous.- A Classification founded on this View. - A Classification independent of Theory. - Constituent Minerals of Granite. Varieties of Granite. Structure and Appearance of Granitic Mountains. - Mont Blanc, and the Aiguilles in its Vicinity. Localities of Granite. Granite Veins.. Passage of Granite into Porphyry and Sienite. - Minerals found in Granite. On Granite as the Foundation Rock on which other Rocks are laid. The relative Antiquity of different Granitic Mountain Ranges. Granite pierced through by Porphyry and Currents of Lava. - Granite sometimes protruded among the upper Strata.

In describing the different classes of rock, we may either commence with the lowest or most ancient, or with the uppermost or most recent; but I am persuaded that the student will find it most convenient to begin with the lowest, and proceed in an ascending series to the uppermost. The rocks called primary have distinctly marked mineral characters, and contain few, if any, organic remains. As the student proceeds, he may trace the first indications of organic existence, and in ascending to the upper rocks, he will observe the gradual increase of genera and species that have left their remains in the dif ferent beds; in some cases indicating great changes in the condition of parts of the globe, as from sea to land, or from salt water to fresh, or from deep to shallow seas. If the student begin with the more recent or uppermost strata, he will find them difficult to recognise by fixed mineral characters, and he will be confused by the variety of organic species presented to his notice, but from which he can derive little instruction, until he be able to compare them with the fossil remains in the lower strata. In the geological description of a particular country or district, it may often be more convenient to commence with the beds nearest the surface, and proceed in a descending series, but then the reader is supposed to be already acquainted with the science.

IF any rocks can with propriety be denominated primary or primitive, they are those which are most widely spread over the globe in the lowest relative