308 HANDBOOK OF NATURE STUDY 



tree-like ferns, horsetails, and lycopods, which grew in vast 

 swamps. Here one generation after the other fell into the 

 shallow water, the land was slowly sinking, the salt water 

 of the ocean killed the plants, and the thick mass of debris 

 was covered by clay and sand, which was carried in from 

 the adjacent land. A slow process of decay, and the great 

 pressure exerted upon them, has left nothing of these grand 

 forests but the coal, which was in their trunks and foliage. 

 Geologists believe that millions of years have passed since 

 the coal plants nourished. 



Formations made by the remains of animals. Let us 

 carefully examine these pieces of limestone and marble. 

 We find that they consist entirely of shells and other cal- 

 careous remains ' of animals. We also know that corals 

 form islands and reefs, the latter sometimes being hundreds 

 of miles long. Rock formed by animal remains is lime- 

 stone ; and it has been calculated that the average thickness 

 of this rock over all the continents is about five hundred 

 feet. The white chalk cliffs of England and France are 

 also the calcareous remains of very small animals. 



83. Finally, we must mention the interior heat of the 

 earth as an important factor in determining the character of its 

 surface. Whether the earth's interior is solid or liquid is still 

 a mooted question ; but the high temperature in deep mines, 

 the boiling waters ejected by geysers, and the glowing lava, 

 or melted rock, and steam erupted by volcanoes, is sufficient 

 evidence that a temperature high enough to melt all the 

 rocks, metals, and minerals we know of prevails in the 

 interior of the earth. Volcanoes often eject enough lava 

 and ashes to cover many square miles, and have buried 

 whole cities. Hundreds of square miles in the Eocky 

 Mountains are covered by lava, which very long ago came 

 up through great fissures in the stratified rocks. It is a 



