Slike stranica

In this belt or chaplet of plateaus lie the high tablelands of South and Northeast Africa, Abyssinia, South Arabia, Persia, Beloochistan, North Deccan, Afghanistan, Thibet, East Tangut, and eastern Gobi, in Mantchooria.

Correspondent with this immense plateau belt, in the New World, is the great American chain, once a wholly volcanic, and though differing so much in structure, direction, and hydrographical influence, yet giving the globe a wholeness, a unity in diversity, which is strikingly apparent.

The Origin of Mountains.

The linear regions of elevations of the earth's surface, as we may term them, in contradistinction to the plateaus which are characterized by breadth rather than by length, have been projected in the form of mountain chains, as has been already hinted, through huge fissures made by the rending of the earth's crust. The upheaval to fill the seam has, in some cases, been all made at once; in others, in a succession of periods. The uniform agreement of all the geological strata or their diversity decides this point. Sometimes the rocky strata are laid bare and easily investigated. Often, however, the observer is obliged to draw conclusions from a part to the whole. Yet in all cases the mountain, in contradistinction to the adjacent plateau, is the tract which has been thrust through the crust. The frequent steep and lofty precipices show the immensity of the internal force required to lift the mountains from their places, while the lines of stratification indicate the direction of upheaval. The rifting of a seam in the earth's crust was the first step in the formation of mountains; the filling up of the seam by liquid matter, the second step. The upheaval of Asia, from the Persian plateau to Gobi, in a line 60° N. E., seems to be con


nected with the most ancient revolution which the earth's crust ever experienced. The mountains there are, therefore, more modern in origin than the plateau on which they stand. The direction of the chain, in all cases, seems to have been dependent on the direction of the fissure in the earth's crust, which the mountain range afterward fills. The breaking through the crust necessarily occurred when the pressure beneath the surface was very great, or when a moderate pressure was exerted beneath a thin crust, where the resistance was slight.

The latter case seems to have been prevalent in most plateau regions. Their own gradual upheaval probably thinned the surface, and made it more liable to fracture. This accounts for the fact that the greatest mountains of the globe are found contiguous to plateaus. And the broader the original seam in the crust was, the broader the mountain range which rose to fill it, either at a single upheaval, or in a series of convulsive throes projecting successive masses of molten matter from below. In the latter cases the strata thus formed lie on each other like the leaves of a book, their constitution changing according as the more advanced stages of melting in the vast internal caldron throw out more metamorphosed rocks. These later layers rose to a greater or less height on the sides of the partially-formed mountain, according to their specific gravity, their more or less fluid state, and their rapidity of cooling, as we can now see by examining the layers in their present permanent condition.

Thus far we can conjecture, with great security, taught by the manifestly wild and fierce convulsions which once threw up the mountains, since in them distortion is the rule and regularity of structure the exception, and also by the equally manifest quiet and sustained process of up

heaval, when the plateaus were formed; their strata being in a state of regularity and unbroken repose.

When the great vents produced by the outward pressure of internal volcanic forces occurred beneath the sea, they were filled up in the same manner as on the dry land, excepting that the summits of the mountains emerge above the surface in the form of isolated islands, or when there was a chain or group of mountains upheaved, as an archipelago. When there was no rifting of the surface, and no forcing up of whole chains of peaks through a thinned crust, the fierce action of the internal heat appears to have necessitated the upheaval of solitary volcanoes here and there, in some cases even rows of them, to give vent to the pent-up steam and gases, and to convey away the molten tide within. When such volcanic series rose in parallel ranges, they lifted, or may have lifted up the whole district between them, as if upon their shoulders, and so formed the American type of plateaus, of less breadth and greater length than the Asiatic, and in height corresponding with the volcanic peaks which form their rim, and to which they are probably indebted for the form of their


It needs hardly to be added to what has been said above, that the general direction of existing mountain chains depends upon the direction of the primitive seams made in the earth's surface by internal forces. The Ural Mountains, the Scandinavian chain, the Alleghanies, the Ghauts, run on meredian lines; others more or less transversely.

The various kinds of rock which have been thrown up in mountains enlighten us as to the process and results of the internal heat of the earth; the successive formations display not only the various eruptions of molten matter, and

its discharge in new layers above what had been thrown out before, but reveal the relative age of the various formations. We have in a single chain sometimes a whole volume of history, marking off the epochs of upheaval with the most perfect legibility and exactness. Many crystallized rocks result evidently from the gradual process of cooling after the ancient exposure to the intense heat of the inner portions of the earth-granite, porphyry, gneiss, slates, and the so-called metamorphosed rocks. These used to be considered the oldest formations, but the upheaval theory treats them as the latest formed.

Most mountain chains have been uplifted to their present height by a succession of upheavals. To accomplish this has been labor of uncounted thousands of years. Only a very few-the main Carpathian range, for instance—seem to have been upheaved at a single convulsion, and to have assumed their present appearance at once. Where there were incessant eruptions accompanied with flames, and masses of molten matter (lava) have been ejected from the crest or from single summits, these volcanoes and volcanic ranges have been the result. Elsewhere no such phenomena have been visible. Possibly, in such cases, the masses cooled so rapidly as to extinguish or fill up what may have been embryo craters, and the plutonic acclivities may have been repressed, leaving us the traces of primeval eruptions, but no vestiges of any dangerous forces remaining till now. Of mountains formed in this manner, may be mentioned the Puys de Dome, the Bohemian basaltic peaks, trachyte Transylvania Alps, the Katak Kaumene, ("Burnt Tract,") of Asia Minor, and Hauran, Iceland, parts of the great American chain, parts of the Sunda chain, the South Sea Islands, and Bagdoola, and its range of extinct volcanoes in the Thian-Shan chain.

But other forces besides fire were competent to form mountains and plateaus, to spread layers of clay and sand and various deposits at the bottom of the sea, afterward to harden into strata of rock. In contradistinction to plutonic formations, these have been called neptunic, because formed at the bottom of the sea. The oldest of the neptunic or stratified rocks have been upheaved by the subterranean forces, and now are found in the elevated plateaus or mountain ranges, still having, however, their unbroken irregularity of structure. Also, after the stratification has been complete, and plutonic acclivities have opened the seams in the earth of which I have already spoken, and molten masses have rushed up to fill them, fragments of the primitive stratified rocks have been caught up and raised, together with the molten masses, to the very summits of lofty mountains; so that the geologist finds fossils there more or less perfectly preserved, the stratified rocks which contain them surrounded by the plutonic rock upheaved from below the surface. Chalk layers full of mollusca and infusoria have been found by Humboldt and von Buch on the very summits of the Andes, and corresponding with those which have been. discovered by Ehrenburg in the deposits at the bottom. of the sea.

Other older and more recent oceanic deposits are found in their primitive condition at the bottom of the sea, or in very low places on the land. In such localities the surface of the earth is composed of horizontal or slightly inlined layers or strata, of secondary formation, and whose origin in deposits from water cannot be denied. These are the beds of chalk, clay, sand, marl, gypsum, and other common substances; and these strata again have been overlaid with more recent accumulations, the result of di

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