Mountains help to cause movement and change in the atmosphere. Let us see how this takes place. Mountains expose on one side their masses of rock to the full heat of the sun. Rocks are capable of becoming highly heated under a blazing sun: we have known stone walls, even in England, to be almost too hot to touch; and perhaps the reader may have often noticed the quivering of the hot air as it rises from the ground on a summer day, especially over a road or any piece of bare rocky ground. This quivering tells us that the air is highly heated by the ground beneath, and is consequently rising. You know how the pebbles look beneath a clear running stream; and the things which we see through air in this state all seem to be similarly moving or quivering. It is easy then to imagine how masses of heated air would rise up from the side of a mountain-range which faces the sun,—that is, the southern side,—while on the other, or northern side they cast a soft shadow for leagues over the plains at their feet. In this way mountains divide a district into two different climates, with a light warm air on their southern slopes, and colder air on the northern, and the rising of the warm air will cause a certain amount of circulation and movement. Hence mountains help to make currents in the atmosphere, and these currents produce important consequences.
When mountain-ranges trend more or less directly across the direction of prevailing winds, they always have a moist side and a dry one. In the torrid zone, where easterly winds prevail, the eastern slope is usually the moist side; but in higher latitudes, as, for example, in Europe, the western side of mountain-ranges receives the greatest amount of rainfall, because westerly winds prevail there.
Mountains are barriers dividing not only one nation from another, but separating also various tribes of plants and animals. It will be readily understood that with the exception of birds, whose powers of flight render them independent of physical barriers, most animals find mountains more impassable than men do. We can make roads and railways, but they cannot thus aid their powers of locomotion; hence mountains put limits to their migrations. Still, climate and food supplies have a greater influence in determining the boundaries of zoölogical provinces (see chapter [iv.]).
Mountains are the backbones of continents. A glance at a map of the world will show that there is evidently a close connection between continents and great mountain-chains. This connection shows itself both in the shapes and general direction of continents. Thus, the long continuous line of mountain-chain which extends from the southern spur of the Andes to the northern end of the Rocky Mountains,—a distance of about nine thousand miles,—corresponds with the general trend of the North American continent, and forms the axis or backbone of that vast tract of land. It seems as if the sea on its western side were kept at bay by this great rocky wall, while on its eastern side the rivers have formed new land. A line of mountains is often the coast line, for the sea cannot overcome it unless subsidence takes place. The backbone of Asia and Europe runs east and west, and the continental area of the Old World follows the same general direction.
These are the chief uses of mountains, and the facts which we have brought forward will serve to show how indispensable they are. The following eloquent passage from "Modern Painters" may form a fitting close to the present chapter:—
"And thus those desolate and threatening ranges which in nearly all ages of the world men have looked upon with aversion or with horror, and shrunk back from as if they were haunted by perpetual images of death, are in reality sources of life and happiness, far fuller and more beneficent than all the bright fruitfulness of the plain. The valleys only feed; the mountains feed and guard and strengthen us. We take our ideas of fearfulness and sublimity alternately from the mountains and the sea; but we associate them unjustly. The sea-wave, with all its beneficence, is yet devouring and terrible; but the silent wave of the blue mountain is lifted towards heaven in a stillness of perpetual mercy; and the one surge, unfathomable in its darkness, the other unshaken in its faithfulness, for ever bear the seal of their appointed symbolism:—
"'Thy righteousness is like the great mountains,
Thy judgements are a great deep.'"