1. Mountains produced by denudation.—The extent to which denudation has altered the surface of the globe can scarcely be imagined. All the stratified rocks are produced by its action; but these do not measure its full amount, for many of these beds have been deposited and denuded, not once or twice, but repeatedly before they reached their present state. Masses of rock more indurated, or better defended from the wasting rents than those around, serve as indices of the extent of denudation. The most remark able case of this kind with which we are acquainted, is that of the three insulated moun tains in Ross-shire—Soil Veinn, Coul Beg, and Cool More—which are about 3,000 ft. high. The strata of the mountains are horizontal like the courses of masonry in a pyra mid, and their deep red color is in striking contrast with the cold bluish hue of the gneiss which forms the plain, and on whose upturned edges the mountain-beds rest, ft seems very probable, as Hugh Miller suggests, that when the formation of which these are relics (at one time cOnsidered as old red sandstone, but now determined by sir Roderick Murchison as being older than was first raised above the waves, it covered, with an amazing thickness, the whole surface of the highlands of Scotland, from Ben Lomond to the Maiden Paps of Caithness, but that subsequent denudation swept it all away, except in circumscribed districts and in detached localities like these pyramidal hills.
2. ji/o nta ins produced by internal force.—These are of several kinds. (a.) Mountains of ejection, in which the internal force is confined to a point, so to speak, having the means of exhausting itself through an opening in the surface. The lava, scoria', and stones ejected at this opening form a conical projection which, at least on the surface, is composed of strata sloping away from the crater. Volcanoes are mostly isolated con ical hills, yet they chiefly occur in a somewhat tortuous linear series, on the mainland and islands which inclose the great Pacific ocean. Vesuvius and the other European volcanoes are unconnected with this immense volcanic tract. (b.) But the internal force may be diffused under a large tract or zone, which, if it obtain no relief from an opening, will be elevated in the mass. When the upheaval occurs to any extent, the strata are subjected to great tension. If they can bear it, a soft rounded mountain-chain is the result; but generally one or more series of cracks are formed, and into them igneous rocks are pushed, which, rising up into mountain-chains, elevate the stratified rocks on their flanks, and perhaps as parallel ridges. Thus, the Andes consist of the stratified rocks of various ages, lying in order on the granite and of which the mass of the range is composed. The position of the strata on such mountains supplies the means of determining, within definite limits, the period of upheaval. The newest strata that have been elevated on the sides of the mountain when it was formed, give a date ante cedent to that at which the elevation took place, while the horizontal strata at the base of the mountain supply one subsequent to that event. Thus, the principal chain of tire
Alps was raised during the period between the deposition of the tertiary and that of the older recent deposits. (c.) But there is yet another way in which the upheaving internal force operates, viz., where it does not act at right angles to the surface, but rather obliquely, and, as it were, pushes the solid strata forwards, causing them to rise in huge folds, which, becoming permanent, form parallel ranges of mountains. The crust of the earth, in its present solid and brittle condition, is thus curved, in a greater or less degree, by the shock of every earthquake; it is well known that the trembling of the earth is produced by the progress of a wave of the solid crust ; that the destruction of buildings is caused by the undulation; and that the wave has been so evident, that it has been described as producing a sickening feeling on the observer, as if the land were hut thin ice heaving over water. This mode of mountain formation has been explained when treating of the Appalachians (q.v.), which were thus formed. Many other ranges have had a similar origin, as some in Belgium and in the southern Highlands of Scot land, as has been Suggested by Mr. Carruthers.
It is evident that in the last two classes the parallel ridges were produced at the same time. Elie de Beaumont generalized this, maintaining that all parallel ridges or fissures are synchronous; and on this he based it system of mountain-structure, which is too universal and be.true. The synchronism of parallel had been noticed by it!i4 Vow :reeeiVed its a first prirucfple miring-. The converse is also held to be generally true, that fissures differing in direction differ also in age: yet divergence from a center, and consequent want of parallelism, as in the case of volcanoes, may be an essential charactera,tie of contemporaneity. Nevertheless, Elie de Beaumont ctusaffied the mountains of the world according to this parallelism, holding that the various groups are synchronous. The parallelism does not consist in having the same relations to the points of the compass—for these, as regards n. and a., would be far from parallel—but is estimated in its relation to some imaginary great circle, which being drawn round the globe would divide it into equal hemispheres. Such circles he called great circles of reference. But beyond this, lie went a step further, and proposed a more refined classification, depending on a principle of geometrical symmetry, which he believed he had discovered among his great circles of reference. It is to be feared, how ever, that his geometrical speculations have little foundation in nature.