among clay, many miles from where there are any granite rocks on the surface, and there can be no doubt that they were originally frozen into an iceberg, which floated away with them and when it melted left them so far from their native place. In many of the midland and eastern counties once floated these icebergs, dropping the stones and boulders which they brought away from the Welsh, Cumberland, and Scottish mountains. The climate of the earth must have been fearfully cold when our country was covered with ice, just as Greenland is now. Geologists suppose that there must have been more than one age of ice, and that between these ages the climate of the world was pretty much the same as at present, although it is certain that there were periods when England was much warmer, because many of the fossil plants could not have grown in a cold climate. You will want to know whether there were any land animals living during the ice periods. It is impossible to be quite certain, but it is most likely that the mammoth was living both before and during the _last_ ice age, because its bones have been found among the earths brought down by the glaciers. I have said all you will be likely to remember at present about the nature of the different rocks, but it will help you to understand better how they have been laid one upon the other, and how they have been moved and broken by upheaval and subsidence, if you look at the drawings on page 51. DENUDATION. It has often happened that some of the harder and older rocks, like granite and slate, have pushed themselves through those earths lying above them, and then the sea or a great river has washed away all the earths from one side of the rock. The rain, too, falling for thousands of years, has swept them down into the valleys and mixed them together. This is called denudation, or “laying bare” the harder rocks by washing the softer ones away from them. Those beds of pebbles on the sea shore also have been battering against the rocks for ages and very gradually wearing them away, as you can see if you watch the stones being driven into and sucked out of holes and cracks by every wave. Thus, both the loose stones and the solid rocks get polished and ground away, and Nature is always destroying and making again by turns. If this destruction went on continually without any raising of the land to make up for it, the surface of the whole Earth would in time become level; but old sea beds are always being slowly raised above the water and prepared for the growth of plants and the habitation of animals. [Illustration: _Upheaval._] [Illustration: _Subsidence._] [Illustration: _Denudation._]