Continents and parts of Ocean have, so to speak, changed places. Mountains have risen; and tablelands, high uplifted, have sunk low. Sea-beds have become dry land; and dry lands have become sea-beds. Rivers have carved new paths for themselves; and cataracts have worn away vast masses of rock, carrying the débris to the sea. Ocean-waves have battered down lines of cliffs, and new cliffs have emerged from under water. All these things have come about, not in a few years, but in hundreds of years, in thousands of years, some say in millions of years. Such time-possibilities are not, however, without limits. Sooner or later the Astronomer steps in with a--“Hold! Enough! The Sun as a light and heat giver could not have existed then.” But we may leave such perplexing comparisons and calculations, and may content ourselves with a general--“Very long since!” There was a time when men believed our Earth to be at rest, in the centre of a revolving Universe. That notion had to be given up as knowledge grew. There was a time, far more recent, when we all felt confident that this firm ground, on which we live and move, stand and work, was solidly calm and immovable. That notion too has had to be abandoned. For the Earth-crust itself is in motion; certainly in parts; probably as a whole. Here it is gently heaving upward; there it is slowly sinking downward. True, we do not see or feel such movements. But neither do we feel the whirl of our Earth upon her axis, or her rush around the sun. Neither do we see from hour to hour the growing of a boy into a man, or the change from a sapling into a tree. Straight through from side to side, the Earth measures some eight thousand miles; and of those eight thousand a very slight “skin” is all that we can study, by anything approaching to direct observation. No wells or mines on land can be sunk so deep as the sailor’s plumbline in the ocean. If we add the six miles or more of the ocean’s greater depths to the five miles or more of Earth’s higher mountains, we have at most twelve miles, which, compared with the whole diameter of the Earth, must be looked upon as a mere nothing. By one means we are able to learn something of what lies deeper in the earth-crust. Half a dozen deep wells may be sunk in different places, the same boring apparatus being used, and the same methods being followed. But the same results would not be obtained. Earth’s crust is not one solid continuous substance, like a shell of iron. It is made up of different substances, lying one upon another, or mixed confusedly together. When men work their way downwards, in well-sinking or in mine-sinking, they come across a great variety of layers, each unlike the rest.