in the sea being covered up with clay,--with mud,--and more shellfish living on the top of that; and then, are they, too, being covered up? So that in years to come they will be found in layers of clay and stone like those we have been looking at in quarry and sea cliff? We have asked our questions. Now we must look around, and see if we can find the answers. After it has been raining heavily for two or three days go down to the marshes of the Yar, and stand on one of the bridges over the stream. We have seen it flowing quite clear on some days. Now it is yellow or brown with mud. Where did the mud come from? Go into a ploughed field with a ditch by the side. Down the ditch the rain water is pouring from the field away to the stream. It is thick with mud. Off the ploughed field little trickles of water are running into the ditch. Each brings earth from the field with it. Off all the country round the rain is trickling away, carrying earth into the ditches and on into the stream, and the stream is carrying it down into the sea. Now think. After every shower of rain earth is carried off the land into the sea. And this goes on all the year round, and year after year. If it goes on long enough--? Look a long way ahead, a hundred years,--a thousand,--thousands of years. We shall be talking soon of what takes many thousands of years to do. Why, you say, if it goes on long enough, all the land will be carried into the sea. So it will be. So it must be. You see how the world is changing. You will soon see how it has changed already, what wonderful changes there have been. You will see that things have happened in the world which you never guessed till you began to study Geology. Now, let us go a bit further. What becomes of all the mud the streams and rivers are carrying down into the sea? Look at a stream coming steeply down from the hills. How it rushes along, rolling pebbles against one another, sweeping everything before it, clearing out its channel, polishing the rocks, and carrying all it rubs off down towards the sea. Now look at a river near its mouth in flat lowland country. It flows now much slower; and so it has not power to bear along all the material it swept down from the hills. And so it drops a great deal; it is always silting up its own channel, and in flood time depositing fresh layers of mud on the flat meadow land,--the alluvial flat,--through which it generally flows in the last part of its course. But a good deal of sediment is carried by the river out to sea. The water of the river, moving slower as it enters the sea, has less and less power to sweep along its burden of sand and mud, and it drops it on the sea bottom,--first the bigger coarser particles like the sand, then the mud; farther out, the finer particles of mud drop to the bottom.