262 HISTORICAL OfiOLOGY. 



In the diagram (Fig. 163) the different rock-systems 

 are placed one on top of the other, and the vertical black 

 spaces represent by their breadth the relative dominance 

 of different classes at different times. 



Periods and Epochs. The subdivisions of these again 

 into periods and epochs are founded on more local uncon- 

 formities, and especially on less important changes in the 

 species. 



We have already, 011 page 204, given a schedule of the 

 most important divisions and subdivisions adopted in this 

 work ; but we shall not treat separately all of these. As 

 in human history, so in geology, the earliest times are little 

 known, and are touched lightly. As we come toward the 

 present, and events thicken, we shall take up subdivisions 

 more and more first ages, then periods, and, finally, even 

 epochs. We give here also (Fig. 164) a generalized sec- 

 tion of American strata, which will be found useful for 

 reference. It must not be supposed, however, that all 

 these strata occur in any one place. It is an ideal section, 

 in which all the most important American strata occurring 

 in different places are brought together and arranged in 

 the order of time. 



We are now ready to commence a rapid survey of the 

 history of the earth. But it must be understood that we 

 can commence only where the record commences. Before 

 this is the abyss of the unrecorded, of which we know 

 nothing positive. Before the historic is the prehistoric ; 

 no history can recall its own beginning. 



