chapter viii., page 277). Again, granite, basalt, and other rocks

known as "igneous," which once existed in a molten condition, have forced their way up from subterranean regions into the rocks forming mountain-chains; and a good deal of the hardening just alluded to is due to the presence of these fiery intruders, which have baked and hardened the rocks around them to a considerable extent, altering at the same time their mineral composition. The same causes which led to the injection of granite, basalt, and other igneous rocks in mountain-ranges brought other consequences in their train. Whatever the causes, they were closely connected with volcanic eruptions, so that highly heated water and steam found their way through cracks and other fissures in the rocks; and in the course of time the chemical actions thus set up led to the deposition of valuable metallic ores within these fissures. In this way mineral veins were formed; and volcanic action seems to be largely responsible for the production of minerals. Thus we find around Vesuvius, and in fact in all volcanic regions, large and varied supplies of minerals. Now, the geologist discovers that many mountain-chains--such, for example, as the Grampians, Alps, and Carpathians--have in past geological periods been the seats of volcanic action on a grand scale; and so we need not be surprised to learn that mountainous countries yield large supplies of valuable gems and metallic ores (see chapter viii., page 277). Even in the days of Solomon, the active and business-like Phoenicians were carrying on trade with Great Britain; and the tin came from Cornwall. Besides tin, gold, silver, lead, copper, zinc, and other metals come from our hills. Now, however, we get our copper mostly from the Andes, and our gold from Australia or South Africa, because it can be got more cheaply from these countries, to which many of our Cornish miners have emigrated.

Precious stones also come chiefly from the hills, for the same reason; for they were formed at the same time and by the same causes. Cairngorms, agates, chalcedony, jasper, onyx, topaz, diamonds, and many other gems are silent but certain witnesses to the action of subterranean heat, acting long ago on the rocks which we now see standing up high above the general surface of the ground, though once they were buried deep down below the surface. Diamonds as well as gold are often got from the beds of streams, but this is easily accounted for; the streams have washed them out and brought them down from the hills.

The following words from the Book of Job (xxviii. 5) might well be applied to the hills.

"As for the earth, out of it cometh bread: And underneath it is turned up as it were by fire. The stones thereof are the place of sapphires, And it hath dust of gold."

We have thus explained the three principal services rendered by mountains, but some others remain to be mentioned.

_Mountains have an important influence on climate._ The climate of highlands everywhere has certain peculiarities which distinguish it from that of adjacent lowlands. The air resting on mountains is less dense than that of the lowlands, and hence has fewer molecules to obstruct the entering sunbeams by day, or to stop the outward radiation at night. Therefore mountain air must be cooler; and so we find that on mountains the mean, or average, annual temperature is lower. This rarity of the air causes the ground to become hotter by day and colder by night than the ground of the plains; and so the extremes of temperature are greater. These extremes are injurious to vegetation in the higher regions, and the want of moisture still more so. But mountain-slopes _up to a certain height_ usually have a moist climate; that is, they have more clouds and rain than the surrounding lowlands. Below the region of snow there is generally a heavy growth of forest; and forests in their turn exercise an important influence, helping to collect moisture, and in various ways to prevent extremes either of heat or cold.

The earth is divided into three well-marked zones or belts of climate: (1) The torrid zone within the tropics, where the sun is vertical twice a year, and days and nights are nearly equal; (2) The temperate zones, where the sun's rays come more obliquely, and so are less powerful, and where the length of day and night varies considerably; and (3) The frigid zones, round each of the poles, regions of intense cold, where for six months of the year the sun is never seen. Now, these broad divisions, so familiar to school children, are considerably interfered with by the height of various districts above the sea-level, or, as geographers say, by altitude. High ranges of mountains bring somewhat arctic conditions with them, even in low latitudes, where one would expect great heat. Thus the climate of the plains is very different from that of their neighbouring mountain-ranges, although their latitudes are practically the same. Travellers in Switzerland know how hot it can be in the Rhone Valley or in the plain of Lombardy, and how much cooler it is when you get up among the glaciers and the snowfields. Or to take an illustration from Great Britain: a hot summer would be somewhat trying in Edinburgh, Glasgow, or even Inverness, because they lie low, while among the Grampians, on Speyside, or Braemar, it would be very pleasant.

Vegetation follows climate. The sultry plains of the Ganges show a luxuriant tropical vegetation, while on the middle slopes of the Himalayas the climate is temperate, like that of Europe, and consequently the vegetation resembles that of a temperate region; and the highest parts of this great range are like polar latitudes in their climate, and partly also in their vegetation.

The arctic character of the climate of high mountain regions shows itself in the flora; for on the High Alps and the Highlands of Scotland and Norway, we find no small number of truly arctic plants whose home is much farther north. A very long time ago, when the climate of the whole of Northern Europe was extremely severe, and when great glaciers descended from the mountains into the plains, so that the aspect of the country was somewhat similar to that of Greenland at the present day, arctic plants and animals came down from their northern home, and flourished abundantly. This was during the _Great Ice Age_, which has left behind unmistakable evidences which the geologist can interpret as if they were written records. Then for some reason the climate became milder, the glaciers melted away, in Great Britain at least; but these arctic plants were left behind, and flourished still on the cool mountains, though they died out on the warm plains (see chap. iv., pp. 123-124).

_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.'"