These two words are so often used in books on geology that we shall not be able to get on without knowing their meaning. We have seen that the rocks have been formed in a certain way—some by heat, some by water, and some by dead forests—and that they lie over one another in pretty regular order. But this order has sometimes been disturbed and the layers have been tumbled about among one another very much. In some places the older rocks, such as granite, slate, and sandstone, have been pushed up through those above, and in others the coal has sunk down and been covered with thick layers of chalk, sand, and mud. When the force below pushes a layer up through the others it is called _upheaval_, and when a layer sinks down it is called _depression_, or _subsidence_. Both these actions are going on now in different parts of the Earth. A great part of Sweden, Norway, and Denmark, of Spitzbergen, Siberia, and the north of America, is being slowly raised higher above the sea, as we know by the height their old sea beaches now are above the water; while part of the shore of America opposite to Europe and also the south of Greenland is slowly sinking down, as we know by the remains of land animals and trees which are now covered by the tide; and at many places on the coast of India this subsidence is also going on. Nearer home, too, there is an example of it in the island of Guernsey. All round the coast of this island, like that of Jersey, are found tree trunks and other remains of old forest land beneath the water. Old histories refer to this as dry land; and if a map of it made in 1406 is correct, this land must have sunk about 150 feet since that time. Thus we can see, even at the present time, the very same changes which have worked upon our Earth for innumerable ages. It is now easy to understand how the forests which must have grown above in the air have, after a long time, sunk down to a great depth, and been turned into coal, and covered with the sediment, sand, gravel, and chalk from the seas which afterwards flowed over the places where they grew. Sometimes the rocks by the sea shore are cut into terraces or steps by the constant wear of the water, and when we see these water marks far above the present level of the sea we know that the land must have been lifted up gradually above the sea. There are many such terraces in Norway. To prove whether this is so marks have been cut upon rocks at a measured height above the sea, and after some years these marks have been noticed to have been raised much above the water by the “upheaval” of the earth at that place.