THE BUILDING OF THE EARTH 



come covered with forest, and the forest may sink below 

 the sea, only to be pushed up again and become dry 

 land a million years later. Each of these changes will 

 leave its mark, each will be accompanied by deposits. 

 The deposits may be vegetable matter, trees and mosses, 

 and the growth of swamps, such as coal was first made 

 of; or they may be the ocean sludge, which at last be- 

 came chalk or limestone. 



28 



