HO W EXTINCT MONSTERS ARE PRESER VED. 15 



distance from the shore, is discoloured with mud — especially at 

 the mouth of a river. The sand, being heavy, soon sinks down, 

 and this is the reason why sand-bars so frequently block the 

 entrance to rivers. Then again, the waves of the sea beat against 

 the sea-shore and undermine the cliffs, bringing down great 

 fragments, which after a time are completely broken up and 

 worn down into rounded pebbles, or even fine sand and mud. It 

 is very easy to see that in this way large quantities of sand, gravel, 

 and mud are continually supplied to our seas. We can picture 

 how they will settle down ; the sand not far from the shore, and 

 the fine mud further out to sea. When the rough weather ceases, 

 the river becomes smaller and flows less rapidly, so that when 

 the coarse debris of the land has settled down to form layers, or 

 strata, of sand and gravel, then the fine mud will begin to settle 

 down also, and will form a layer overlying them or further out. 

 Thus we learn, from a little observation of what is now going on, 

 how layers of sand and mud, such as we see in a quarry, were 

 made thousands and thousands of years ago. 



When we think of all the big rivers and small streams con- 

 tinually flowing into the sea, we shall begin to realise what a 

 great work rain and rivers are doing in making the rocks of the 

 future. If, at a later period, a slight upheaval of the sea-bed were 

 to take place so as to bring it above water, and such is very likely, 

 these materials would be found neatly arranged in layers, and 

 more or less hardened into solid rock. 



The reader may, perhaps, find it rather hard at first to 

 realise that in this simple way vast deposits of rock are being 

 formed in the seas of the present day, and that the finer 

 material thus derived from a continent may be carried by 

 ocean currents to great distances ; but so it is. Over thousands 

 of square miles of ocean, deposits are being gradually accumu- 

 lated which will doubtless be some day turned into hard rock. 

 Just to take one example : it has been found that in the 

 Atlantic Ocean, a distance of over two hundred miles from the 



