We have seen now what the principal rocks are made of and the way in which their places have been changed by upheaval and depression. Water, as we know, has been at work and has done great things in _all_ ages of the World’s history. I have called it “one of Nature’s most powerful tools,” and when we look at the quantity of chalk alone that there is in the world, and remember that this was all laid down in water, and perhaps a great part of its lime carried down by rivers to the seas where it settled to the bottom, after the corals and small shell-fish had worked it into their bodies, we are right in thinking water a great Magician indeed. Why, even so small a river as the Thames carries down to the sea every year as much dissolved earth as would make a good large hill; and what must such rivers as the Nile, the Amazon, the Mississippi, and the great Chinese rivers do! There must have been gigantic rivers, too, in the old times, or else it would have been impossible that the deep sandstone and slate beds could have been formed; for these are all laid down by the washing away of earth in water. Ice, which is only solid water, has also been a powerful tool in shaping the surface of the Earth, but it has not been _always_ at work as water has. Ice now covers only a comparatively small part of the globe near the north and south poles, and mountains like those in Switzerland; but by watching what ice is doing now in these places we are able to be certain that there has been a time when it covered Scotland, Cumberland, Wales, Sweden and Norway, and nearly all North America. In watching the great “rivers of ice,” called glaciers, in the Alps, for instance, we see that they slip down from the mountains slowly, creeping on year by year, and bringing with them pieces of rock and stones. We see also where they have melted that they have been grinding the rocks beneath them with their great weight, and have cut grooves into, and scraped and polished the hardest granite. The stones underneath the glaciers have been pressed so heavily upon the rocks that they have left deep marks, and we find the same kinds of marks and heaps of stones in many mountains where there are no glaciers now. There are other things too which convince us that a great ice sheet spread over almost the whole of Great Britain. When the huge icebergs break away from the frozen shores of Greenland and North America, they often have frozen into their ice large blocks of rock, sand, gravel, &c., and when they drift into the warmer seas of the south they melt, and of course these blocks or “boulders,” as they are called, sink to the bottom. Just the same kind of boulders are found in many parts of the world, where icebergs never come now, and as they are of a different rock from that on which they lie, they must have been brought there somehow. We naturally suppose then that they were brought by icebergs. Sometimes boulders of granite have been found thus