184 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 ar- 

 ranged one above the other in a horizontal manner, like 

 the leaves of a book, the lowest being the oldest. In conse- 

 quence of the commotions which the crust of the globe 

 has undergone, many points of its surface have been eleva- 

 ted to great heights, in the form of mountains ; and hence 

 it is that fossils are sometimes found at the summit of the 

 highest mountains, though the rocks containing 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 succession, at the 

 surface, or on the slopes of mountains, as seen in the dia- 

 gram (Fig. 154). 



460. The sedimentary rocks are the only ones which 

 have been found to contain animal and vegetable remains. 

 They are found imbedded in the sediment, 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 nume- 

 rous. The comparison and detailed study of them belongs 



* 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 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 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 now (Fig. 154, T,L). 



