216 GEOLOGICAL SUCCESSION OF ANIMALS. 



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

 slates, marls, or grits, according to their chemical and me- 

 chanical composition, and contain the remains of the animals 

 and plants which were scattered through the waters.* 



459. The different strata, when undisturbed, are arranged 

 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 undergone, 

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

 have been elevated to great heights, in the form of moun- 

 tains ; and hence it is that fossils are sometimes found at the 

 summit of the highest mountains, though the rocks contain- 

 ing them were originally formed at the bottom of the sea. 

 But even when folded, or partly broken, their relative age 

 may still be determined by an examination of the ends of 

 the upturned strata, where they appear or crop out in suc- 

 cession, at the surface, or on the slopes of mountains, as seen 

 in the diagram, (Fig. 154.) 



460. The sedimentary rocks are the only ones which have 

 been found to contain animal and vegetable remains. These 

 are found imbedded in the rock, just as we should find them 

 in the mud now deposited at the bottom of the sea, if laid 

 dry. The strata containing fossils are numerous. The com- 

 parison and detailed study of them belongs to Geology, of 



* 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. 154, 

 M,) being probably sedimentary rocks, which have undergone consider- 

 able changes. The Plutonic rocks, as well as the metamorphic rocks, 

 are not always confined to the lower levels, but they are often seen rising 

 to considerable heights, and forming many 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 at the present era, (Fig. 15-4, T. L.) 



