Generally this change of level has taken place gradually, and the greatest work in moving the layers of earth and displacing them has been very slow. But in some places violent and sudden shocks have happened, tearing up the rocks and piling them up in heaps; and now and then islands have suddenly appeared in the sea and vanished out of sight completely in a short time. Islands have thus come up in the Mediterranean Sea within the memory of man. In the year 1831 the island of Julia suddenly appeared near the coast of Sicily, and since the year 186 B.C. no less than three islands have started up in the bay of the island of Santorin. In this century islands have appeared among the Azores, the Indian Archipelago, the Philippines, the Moluccas, and on the coast of Kamtschatka and other places. Some of these have appeared suddenly, others slowly, and they no doubt have been raised by a great force from below. You will see now how easy it is to account for the changes of the places of the layers of rock. The same thing is going on now which has been going on throughout all time, only perhaps with more energy formerly than now, making mountains, islands, and continents, raising up a large tract of land in one place and sinking an island or a sea shore in another. These changes have been of great use to us too. Suppose all England had been covered with coal or slate, we should not have been able to grow anything! As it is we have sand and gravel in one county, chalk in another, slate or granite in another, and coal down below in several, and we can grow a great variety of plants on all these different soils. We have to thank “upheaval” and “depression” for this. The force which is always working below us has turned up the different soils like a gigantic plough, and brought some to the top and covered others, so that instead of having to dig down deeper than ever we have yet, we have only to go from one county to another to find the different rocks. We know that we could not get at the coal in Sussex without going down an unknown depth through the chalk and other earths, but we dig for it in the North of England, where we know its depth below the surface.