38 



INTRODUCTION. 



merged beneath the waters. In the lower strata of the series it is to be observed, that whole 

 genera and species of animals are found which do not now exist in the known world, but 

 as we rise in the series, we find some genera, and at last some species, identical with those 

 now in existence. Yet even tliese show great changes to have taken place ; for it is com- 

 mon to find the remains of tropical plants and animals beneath the surface of countries in which, 

 on account of the coldness of the climate, they could not now live. 



3. Jlge oj Rocks. The stratified rocks having been deposited from water, it is obvious 

 that the lowest in the series must have been first deposited, or in other words are the oldest 

 formations, and that the upper beds are the more recent. No such chronological data are 

 afforded by the unstratified rocks, though their relations to stratified materials aflord just infe- 

 rences respecting their true era. Thus if the latter are wholly below all the strata, we infer 

 that they of course existed before any of the stratified rocks were deposited upon them ; but 

 if they form lines penetrating some of the strata, and overlying them, then the latter had 

 assumed their present condition before the melted mass was poured into its actual position. 

 Such lines of crystaline rock running through the stratified rocks, are called veins or dikes^ and 

 are of common occuri'ence. Superposition of strata is, then, the chief" indication of the rela- 

 tive age of rocks ; but as certain organic fossils have been found to be peculiar to certain 

 formations, it is often easy to identify particular rock-formations by their included organized 

 remains, and when the order of superposition cannot be readily ascertained, the nature of these 

 remains will serve to determine the geological age of the rocks. It is on such a basis that the 

 received geological classification of strata has been established ; and geologists usually divide 

 them into three great classes, the primary, the secondary, and the tertiary. 



4. Primary Rocks. The primary strata are such as do not include any organic bodies. 



They are the lowest in the series of 

 strata, and are found to repose on 

 crystaline unstratified rocks, general- 

 ly on granite, which, as far as the 

 crust has been penetrated, has the 

 appearance of being the general fun- 

 damental rock. The name primitive 

 was originally given to these rocks, 

 because they were supposed to have 

 been deposited before the creation of 

 organized bodies ; but this cannot be 

 proved, since the cause which crys- 

 talized them, or their fusion, may 

 have occasioned the disappearance of 



organic remains. 



5. Secondary Rocks. The secondary strata are generally less crystaline in structure, and 

 in their lower beds the organic remains are few, being chiefly marine productions ; but these 

 increase in number and variety in the upper beds, which end with the chalk formation. The 

 organized bodies are chiefly marine productions, as zoophytes, Crustacea, shells, and fishes ; 

 reptiles, amphibious animals, and plants, are also found in secondary strata. During the depo- 

 sition of the secondary strata the land appeals to have been gradually rising above the level of 

 the waters which once covered it ; and during the same period there are marks of more sudden 

 changes or disturbances of the strata, which appear to have been owing to the forcible irrup- 

 tion of melted rocks from the bowels of the earth. 



n. Tertiary Roch-f. Above the cretaceous or highest secondary strata, another system of 

 deposits appears, which has received the name of the tertiary strata. These have been in 

 part deposited from fresh water, by which circumstance and by their organic remains they are 

 distinguished from the secondary strata. There are, indeed, some fresh-water animals found 

 in the latter, but these are rare, and the formations that include them, seem to have been made 

 m the estuaries of great rivers. But decided alternations of fresh and salt water deposits are 

 characteristic of the tertiary period. All the fossils of the secondaiy rocks belong to extinct 

 species, and are distinct from those of the tertiarv strata. In the latter, mannnalia first appear, 

 and there are a few species identical with those now existing. 



7. Dilu'jial Formation. In many countries vast tracts are covered with a confused mass 

 of ualer-worn rocks, gravel, and a mixiure of other fragmentary substances, which completely 



Primitive Hocks. 



