HO W EXTINCT MONSTERS ARE PRESER VED. 1 5 
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 
