220 



EIGHTEENTH CENTURY. 



PT. III. 



rocks are being made now, how rivers and glaciers are 

 carrying down earth and stones from the mountains into the 

 sea, and how volcanoes are throwing out melted matter 

 which cools down into hard rock ; and then they must com- 

 pare these with the older rocks in the crust of the earth, and 

 see whether they were not formed in the same way. 



Aqueous (or water-made) Rocks. — When we find a piece 

 of marble made up almost entirely of oyster and other shells, 

 and of pieces of coral, we cannot doubt that it must once 

 have been a heap of loose shells and corals such as we now 

 see on the shore or under the water, and that it has since 

 been hardened into limestone. When we find that by 

 crushing or scraping sandstone we can turn it into sand 

 like that which we see on the seashore, and which we know 

 has been made by the sea grinding the stones and rocks of 

 the beach against each other, then we cannot doubt that 

 the sandstone has once been loose sand, and before that was 

 part of a rock which has been ground down by the waves. 



And so we are led to the conclusion that the rocks of 

 our earth, as we see them now, have been formed out of the 

 materials of still older rocks which existed before them, and 

 are being gradually moulded into other and newer rocks, 

 which will exist when these have been destroyed. Our solid 

 earth is being wasted every day. The sides of the moun- 

 tains are washed down and their materials are carried through 

 the valleys by the running water. In this way the soil is 

 brought down to the coast, and here it is eaten away by the 

 waves of the sea, and falls to the bottom of the ocean, out 

 of which it will be raised again by earthquakes, volcanoes, 

 and other movements of the earth's crust, such as can be 

 proved to be going on in parts of the world at this day. As 

 far back as investigations and reasoning can go we find 



