And, lastly, we have seen that even where such causes might seem at first almost indispensable,--when mountains tell us of mighty internal forces crumpling, folding, and fracturing their rocky framework,--yet even there we can account for what we see without supposing them to have been torn and tossed about by any very violent convulsions. [Illustration: MOUNTAIN IN THE YOSEMITE VALLEY.] Although the question of the cause, or causes, of earth-movements, whereby continents are upheaved, and the contorting, folding, and crumpling of the rocks of mountains produced, is not at present thoroughly explained, it may perhaps be worth our while to consider briefly some of the views that have been put forward on this difficult subject. The words "upheaval" and "elevation," in reference to movements of the earth's surface, are somewhat misleading, but are used for want of better terms. They would seem to imply that the force which produced mountains was a kind of upward push; whereas, in most cases, and perhaps in all, the force, whatever it was, did not act in an upward direction. So it should be understood that we employ these terms only to indicate that the rocks have somehow been carried up to a higher level, and not as suggesting _how_ the force acted by which they were raised. It seems pretty clear that in the case of mountain-chains, at least, the force acted in a horizontal direction, as a kind of side-thrust. This we endeavoured to illustrate in chapter ix. by means of a simple experiment with a sheet of paper; and it was shown how folds similar to those of which Mont Blanc is composed could be imitated by simply pressing the sides of a sheet of paper inwards with one's two hands as it lies on a table. Such lateral pressure, it is thought by many, must be caused by the shrinking of the lower and hotter parts of the earth's crust as they cool, leaving the outer crust unsupported, so that it gradually settles down onto a smaller surface below, and in so doing must inevitably be wrinkled and throw itself into a series of folds (see chapter vi., page 204). The interior of the earth is hotter than the outside; and since there is good reason to think that the whole earth was once upon a time in a highly heated and perhaps half molten condition, we are compelled to believe that it always has been, and still is, a cooling globe. Now, almost all known substances are found to contract more or less on cooling; and so if the materials of which the earth is mainly composed are at all similar in their nature and properties to those which we find on its surface, it follows that the earth must be contracting at the same time that it is cooling, just as a red-hot poker will contract on being taken out of the fire.