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]).
SNOW ON THE HIGH ALPS. From a Photograph by Mr. Donkin