This explanation is accepted by many authorities. It does not exactly imply that mountains were upheaved by earthquakes; but it means that the same forces that elevate continents, heaving them up out of the sea into ridges and very low arches, have been at work to crumple and fold their rocks in some places into stupendous folds, such as we now find form part of the general structure of mountains; and that in so doing they caused fearful strains, too great for the rocks to bear, so that they split over and over again, and in so doing produced jars and shocks that must have been very similar to, if not identical with, earthquake shocks as we know them at the present day. Such an explanation is in striking harmony with what we have already learned about the operations of Nature. It was from the long-continued operation of rain and rivers that the materials now forming mountains were transported to the seas in which they were slowly formed. It was also by the ordinary operations of frost, heat and cold, snow and ice, streams, rain, and rivers that the mountains received their present shapes (see chapters v. and vii.). And now we learn that the gigantic work of upheaval took place in a tolerably quiet and uniform manner,--with perhaps only an occasional catastrophe of a more violent kind, but still according to the same law of uniformity which is the very basis of modern geology, and by means of which so much can be explained. We could give other proofs of the gradual elevation of mountains if they were wanted. But at least enough has been said to give the reader a glimpse into the methods employed by geologists in endeavouring to explain how mountains were upheaved; and to show that it is only by a careful study of all that is taking place now on the earth that we can ever hope to solve the difficult questions that present themselves to all who study those stony records on which the earth has written for our enlightenment the chapters of her ancient history. In conclusion, it may be asked what is the nature of the force that accomplishes all this titanic work of upheaval. Although the question has been much discussed, and some very ingenious suggestions brought forward, we cannot say that any of them are entirely satisfactory. But we know that the earth is a cooling body which loses so much heat every year; and it may be that the shrinking that takes place as it cools, by leaving the crust of the earth in some places unsupported, causes it to settle down, to adapt itself to a smaller surface below, and in so doing it would inevitably throw itself into a series of folds, or wrinkles, like those on the skin of a dried apple. Many think that mountain-ranges may be explained in this way. CHAPTER VII. HOW THE MOUNTAINS WERE CARVED OUT. And surely the mountain fadeth away, And the rock is removed out of its place, The waters wear away the stones: The overflowings thereof wash away the dust of the earth.