3 o ELEMENTARY SCIENCE 



land to-day that there was when man first began to live 

 on earth. There seems to be no danger of being swallowed 

 up by the sea. 



Mountains Worn Down and Seas Filled Up. Though 

 the amount of dry land may have changed very little since 

 human history began, we must remember that human his- 

 tory is a very small thing as compared with the whole his- 

 tory of the earth. What we see happening to-day to the 

 surface of the earth is but a trifle compared with what has 



FIG. 7. Diagram of layers of rock that were laid down in the sea and are now found 

 miles away from the sea. After SNYDER. 



happened in the past. Whole ranges of mountains have 

 been worn down by the ceaseless work of water, and have 

 been carried off, grain by grain, into the sea. Seas and 

 lakes have been filled with this waste of the land, and geol- 

 ogists, who study the rocks and find in them a history 

 of the earth, make maps which show that the continent 

 of North America was once just a series of long and narrow 

 islands. The waste of these islands was carried down into 

 the shallow seas that surrounded them. The mud was 

 pressed down by the weight of more mud. The dead bodies 

 of small animals that lived in that ancient sea were buried 

 in this mud. It hardened into rock. After a great while 

 it was lifted above the surface, so that to-day, hundreds of 

 miles from the sea, we find layers of rocks that were formed 



