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.