PRESERVATION a] 
the fine mud will begin to settle down also, and will form a 
layer overlying them or further out. Then 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 accumulated which will 
doubtless be some day turned into hard rock. Just to take one 
example: it has been found that in the Atlantic Ocean, at 
a distance of over two hundred miles from the mouth of that 
great river, the Amazon, the sea is discoloured by fine 
sediment. 
There is another kind of rock frequently met with, the building 
up of which cannot be explained in the way we have pointed 
out, and that is limestone. This rock has not been deposited as 
a sediment, like clays and sandstones, but geologists have good 
reasons for believing that it has been gradually formed in the 
deeper and clearer parts of oceans by the slow accumulation of 
marine shells, corals, and other creatures, whose bodies are 
partly composed of carbonate of lime. This seems incredible 
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