INTRODUCTION. 



37 



skeleton of Megatherium. 



Skeleton of Fossil Elk. 



distribution into layers or strata, shows that the masses so arranged have been formed by 

 aqueous deposits, or the gradual throwing down of earthy particles from water. When the 

 stratified rocks are crystaline, we are led to infer that they have been, at least partially, melted 

 by fire, since they were deposited by the waters in their existing order. All these processes 

 may still be seen going on at the mouths of rivers, around springs, and in volcanic regions, but 

 on a smaller scale than in former ages of the world. 



2. Organic Remains. Almost 

 all the stratified rocks contain the 

 remains of organic beings, animal 

 or vegetable, or both, and it is in 

 these lelics of the past, that the 

 geologist reads seme portions, a» 

 least, of the former history of tho 

 earth, in peiiods long anterior to 

 tradition, or even the existence of 

 man on this globe. I'lants, shells, 

 and corallines, were first noticed 

 as enteiing into the composition 

 of rocks, and next the remains of 

 fishes and of other vertebrated 

 animals, wcve distinctly recog- 

 nised. These relics are some 

 times of marine and sometimes of 

 fresh-water origin, and sometimes 

 are the productions of the land. 

 The rocks, in which they are 

 contained, and which are now 

 often at great depths beneath the 

 surface, — covered by a nu- 

 merous series of strata, several 

 thousand feet in thickness, - 

 once formed the sm-face of the 

 earth's crust ; the lower rocks in 

 the series will generally be found 

 to contain marine remains ; the 

 stratum abo^ e will, perhaps, con- 

 tain fresh-water shells ; above this 

 we sliall i)erhaps find plants or the 

 remains of huge saurians or lizards, 

 indicating vast swamps or marshes ; 

 higher up we shall meet with quad- 

 rupeds V. hich occupied dry land ; 

 in this way we may show that the 

 spot in wliiih these discoveries 

 are made, lias been successively 

 tlie bottom of the sea, the bottom 

 of a lake or river, a marsh or 

 swamp, higli and dry land. Some- 

 times these remains alternate with 

 each otiier, so that, for instance, 

 the layers will present marine re- 

 mains, then fresh water, then ma- 

 rine again, then land, then marine 

 or fresh water, and so on ; proving 

 that the surface has been some- 

 times raised and sometimes de- 

 pressed, so as to emerge from the 

 ocean, become the habitation of 

 other creatures, and again be sub 



j'lti^i.osinirus, Ichthyosaurus, Ptcrodactylus, Fossil I'lavt!;, >>. r. 



Fossil Tnptr, Lrrphtoelnn, Dog. Crocodile, ^-r. 



