STRUCTURE OF THE EARTH'S CRUST. 391 



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 

 stupendous length of time they have required, it is necessary 

 to sketch the principal outlines of geology. 



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

 are of two kinds : 



1. The Massive R,ocks, 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. 



647. 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 layers 

 or strata have become, as they hardened, limestones, slates, 

 marls, or grits, according to their chemical and mechanical 

 composition, and contain the remains of the animals and plants 

 which were scattered through the water s.f 



648. The different strata, when undisturbed, are ar- 

 ranged one above the other in a horizontal manner, like the 

 leaves of a book, the lowest being the oldest. In consequence 

 of the commotions which the crust of the globe has under- 

 gone, the strata have been ruptured, and many points of the 



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

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



f Underneath the deepest strata containing fossils, between these and the 

 Plutonic rocks, are generally found very extensive layers of slates without 

 fossils (gneiss, mica-slate, talcose-slate), though stratified and known to 

 the geologist under the name of Metamorphic Rocks (fig. 376, M}, being 

 probably sedimentary rocks which have undergone considerable changes. 

 The Plutonic rocks, as well as the metamorphic rocks, are not always con- 

 fined to the lower levels, but they are often seen rising to considerable 

 heights, and forming mary of the loftiest peaks of the globe. The former 

 also penetrate, in many cases, like veins, through the whole mass of the 

 stratified and metamorphic layers, and expand at the surface ; as is the case 

 with the trap dykes, and as lava streams actually do now (fig. 376, T. L.) 



