STRUCTURE OF THE EARTH'S CRUST. 183 



the shields of the Crustaceans, and sometimes the wing-cases 

 of Insects. Most frequently they have lost their original 

 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 bodies of 

 former epochs, obtained from the earth's crust. 



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 dis- 

 tribution of animals in time, is of no less importance than 

 the geographical distribution of living animals, of which we 

 have treated in the preceding chapter. To obtain an idea 

 of the successive creations, and of the stupendous length of 

 time they have required, it is necessary to sketch the prin- 

 cipal 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 some- 

 times 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 of granite, porphyry, basalt, &c. 



2. The Sedimentary or Stratified Rocks, called also 

 Neptunian 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. 



