THE ROCKS AND THEIR STORY 3 



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. 



During the exploring cruise of the Challenger, under the 

 direction of Sir Wyville Thomson, in 1872-6, the most 

 extensive exploration of the depths of the sea that has 

 been made up to the present time, it was found that 

 everything in the nature of gravel or sand was laid down 

 within a very few miles, only the finer muddy sediments 

 being carried as far as 20 to 50 miles from the land, the 



