In the great series of stratified rocks forming what is commonly called the crust of the earth (an unfortunate term which has survived from the time when the interior of the earth was generally believed to be in a fiery molten condition, and covered by a thin coating of solid rock at the surface), there are besides the sandstones, of which we have just spoken, great deposits of dark-coloured clays, shales, and slates. All these can be accounted for by the geologist. They are simply different states of what was once soft mud. The slates tell us that they have been subjected to very severe pressure, which squeezed their particles till they were elongated and all arranged in one direction, and this is the reason why they split up into thin sheets. Others, again, represent vast deposits of carbonate of lime, thousands of feet thick and now occupying hundreds of square miles of the earth's surface. Limestone rocks are as abundant in our own country as the sandstones, shales, or slates. The chalk of which the North and South Downs are composed is a familiar example. It is seen again forming Salisbury Plain, in Hampshire and the Isle of Wight, and then it may be traced running up the country in a long band through the counties of Oxford, Cambridge, Lincoln, until it reaches the coast at Flamborough Head in Yorkshire. Then we have the Bath Oölites so much used in building, for they form an admirable "freestone" that can be easily carved and cut in any direction (hence the term "freestone"); and lastly, the great mountain limestone so well developed in South Wales, Yorkshire, and the Lake country. All these were slowly built up at the bottom of the seas which existed in past ages; great beds of gravel formed at the mouths of rivers, and long banks of pebbles and rounded stones collected on the shore of primeval seas, and were ground against each other as now by the action of the waves, until all their corners were rubbed off. Pebble-beds, called by geologists conglomerates, are met with among the stratified rocks; and their story is easily read by studying what takes place at the present day on our seashores. [Illustration: CHALK ROCKS, FLAMBOROUGH HEAD. FROM A PHOTOGRAPH BY G. W. WILSON.]