Any elevation rising from a base more than 1,000 feet may fairly be termed a mountain, and solitary mountains are usually volcanic, because eruptive rock does not produce chains of mountains. The origin of mountains is probably due to the contraction and compression of the crust of the earth—not merely the surface, but the whole thickness between us and the supposed molten interior. Mountains did not exist from everlasting, for the very good reason that they are (in most cases) composed of stratified rocks. Stratified rocks are sedimentary rocks, and must have been deposited below water, and hardened long before they were thrust up by pressure. Moreover, we find (as has already been explained) shells and remains of marine animals on the higher summits, which prove to demonstration that these mountains are composed of rocks which were laid down under the sea.
Professor Dana was one of the first geologists to advance the theory that contraction and lateral displacement are the causes of the elevation of mountains. A very good illustration of this theory was made by Chamontier, who covered an india-rubber balloon with a thick layer of wax, and when it had hardened sufficiently he pricked a hole in the bladder, which immediately contracted, and the wax at once rose up into tiny similitudes of mountains, showing in a sufficiently clear manner that such protuberances may be produced by the pressure of the earth’s contraction, and in such a mass as our earth the elevations would naturally be very great.
Professor Geikie has shown how, by a very simple experiment, the contortion of mountain strata is effected by pressure. A number of cloths or towels placed flat on a table represent the sedimentary rocks. Place a board with a weight on the top, and the towels will remain flattened. But by holding two boards at the sides and pressing them together (the weighted board still remaining), we shall find the towels crumpled and upheaved like the Jurassic strata shown in the illustration (fig. 712). Professor Heim calls the central masses wrinkles of the earth’s crust. So the Alps were pressed up or heaved into the air, the weather—rain, frost, snow, and sunshine—imparting the infinite variety of “Horn,” “Needle,” and “Peak,” so expressively applied in Alpine nomenclature;—the Matterhorn, Wetterhorn, Weisshorn; the Pic du Midi, Aiguille de Dru, Aiguille Verte, and many other mountains in well-trodden Switzerland will occur to the reader at once.
Fig. 712.—Anticlinal and synclinal curves of the Jura Mountains.
The slopes of mountains—though to the casual observer they may appear very much the same—are very different. We sometimes find a long, easy ascent, —more usually a steepish inclination, perhaps 20°;—in other places, such as on the Matterhorn, an almost perpendicular face. Forty-five degrees rise is very steep, and 53° is the limit of any great mountain’s slope. Cliffs and precipices there are, of course; witness the terrible fall from the Matterhorn to the glacier below—thousands of feet with one tremendous leap from the rock to the ice underneath; but mountain slopes are not precipices. As a rule, we find that one side of a mountain chain is steeper than the opposite one. It is harder to climb up from Italy to Switzerland than to ascend in the opposite direction. The Pyrenees are also steeper on the south side. The Scandinavian mountains likewise are steeper in the west. The Himalaya are steepest towards the sea, so are the Ghauts. We here find a difference between the slopes of the New and Old Worlds. In the former we have the less precipitous mountain slopes towards the east; in the old world they are towards the north, and an inspection of a physical map of the world leads us to the conclusion that the Atlantic and Pacific Oceans are the boundaries of entirely different degrees of slopes. The Pacific and Indian Oceans would appear to border the more precipitous mountain sides; the Atlantic and its connections those less steep.
As a rule, we have the most elevated portions of the earth, mountains, and high tablelands, in equatorial regions; and within the torrid zone every terrestrial climate is to be found, owing to the snows of the high mountains and the heat of the valleys, which are naturally closely connected with the upheaval of mountain ranges. We have already spoken of the never-ceasing influences of the air and water upon the rocks, and we need say little about valleys. There are valleys of dislocation, denudation, and undulation. The great valley of Western Asia, wherein lie the Caspian and Aral seas, seems to have been caused by the upheaval of the Caucasus and the Persian plateau.
Plains are very varied. We have European Heaths and Landes; American Savannahs, Prairies, and Pampas; Asian Steppes, and African Deserts. All of these possess certain features in common, more or less vegetation, and sometimes absolute sterility.
Plateaus, or Tablelands, are elevated plains frequently undulating in character. The Plateau of Bolivia is 13,000 feet high, and extends along by the Andes. The tableland of Quito is nearly 10,000 feet high, and borders on the giants Cotopaxi and Chimbarazo.
