STETTCTURE OF THE EARTHS CRUST. 215 



chemical composition, and are changed into stone ; and 

 hence the name of petrifactions or fossils, under which lat- 

 ter term are comprehended all the organized oodles of 

 former epochs, obtained from the earth's crust. Others have 

 entirely disappeared, leaving only their forms ai d sculpture 

 impressed upon the rocks. 



456. The study of these remains and of their position in 

 the rocks constitutes PALEONTOLOGY ; one of the most essen- 

 tial branches of Zoology. Their geological distribution, or 

 the order of their successive appearance, namely, the distri- 

 bution of animals in time, is of no less importance than the 

 geographical distribution of living animals, their distribution 

 in space, of which we have treated in the preceding chapter. 

 To obtain an idea of the successive creations, and of the stu- 

 pendous length of time they have required, it is necessary to 

 sketch the principal outlines of Geology. 



457. The rocks* which compose the crust of our globe 

 are of two kinds : 



1. The Massive Rocks, called also Plutonic or Igneous 

 Rocks, which lie beneath all the others, or have sometimes 

 been forced up through them, from beneath. They were 

 once in a melted state, like the lava of the present epoch, 

 and on cooling at the surface formed the original crust of the 

 globe, the granite, and later porphyry, basalt, &c. 



2. The Sedimentary or Stratified Rocks, called also Nep- 

 tunic Rocks, which have been deposited in water, in the same 

 manner as modern seas and lakes deposit sand and mud on 

 their shores, or at the bottom. 



458. These sediments have been derived partly from the 

 disintegration of the older rocks, and partly from the decay 

 of plants and animals. The materials being disposed in 



* Rocks, in a geological sense, include all the materials of the earth. 

 the loose soil and gravel, as well as the firm rock. 



