Along the burns and the seashores, and in stone quarries, we often see banks of clay. Usually this clay is full of stones. In some places the clay is merely the softened, crumbling top of the rock, and the stones in it are of the same kind as the solid rock below. In other places the clay contains stones which are quite unlike any rocks in the neighbourhood. Sometimes these stones are very large, and they must have been carried from some distant place, for they are of a kind of rock which is not found in the islands. What is the history of this clay with travelled stones, or “boulder clay,” as it is called? Boulder clay may be recognized by several marks. It is tough and sticky; it shows no bedding or layers; and it may be only a few inches thick, or it may form cliffs thirty or forty feet high. Pick a few stones out of it: you will notice that they are not all of the same kind. Wash them carefully in the sea or the burn. Their ends are blunt and worn, but they are not rounded like sea-pebbles. Their surfaces are smooth, and are covered with fine scratches, as if some one had drawn a needle or the point of a knife along them. Nowhere except in this clay will you see stones with these curious scratches. If you find the place where the bottom of the clay rests on the hard rock, you should carefully remove a little of the clay and lay bare the rock surface beneath. Wash it with a little water, and you will see that it is covered with fine scratches exactly like those on the stones. Now this smoothing and scratching of the stones and of the rock might be explained by imagining that the clay at one time was in motion, pushed forward by some immense force, and that the stones rubbing on one another and on the rocky floor produced these scratches. Among the Alps, in Norway, in Greenland, and in other places where there are high snow-clad mountains or a very cold climate, the snow gathers in the valleys till it forms thick masses, and is compressed by its own weight into ice: these masses are known as glaciers. Glaciers are really slow-moving rivers of ice; they slip very slowly down the valley slopes, travelling usually only a few inches in a day. When they reach the warmer region at the base of the mountains, they melt away, leaving behind them heaps of clay which they have swept down from the hills. The stones in this clay are worn and smoothed and scratched exactly like those in the boulder clays of Orkney and Shetland, and the rocks over which the glaciers have moved are smoothed and scratched likewise.