HISTORICAL GEOLOGY. 137 



mind should attempt to define precisely where the slopes of the moun- 

 tain end, as they pass into the plain around, it might be greatly em- 

 barrassed. It is not in the nature of history to be divided off by 

 visible embankments ; and it is a test of the true philosopher to see 

 and appreciate the commencements and culminations of phases, or of 

 the successive ideas, in the system of progress, amid the multitude of 

 events and indefinite Mendings that bewilder other minds. 



We note here the following important principles : — 



First. The reality of an age in history is marked by the develop- 

 ment of some new idea in the system of progress. 



Secondly. The beginning of the characteristics of an age is to be 

 looked for in the midst of a preceding age ; and the marks of the 

 future coming out to view are prophetic of that future. 



Tliirdly. The end of an era may come, either after the full culmi- 

 nation of the idea or phase, or, earlier, at the commencing prominence 

 of a new and grander phase in the history. It may be as ill-defined 

 as the beginning, although its prominent idea may stand out boldly to 

 view. Thus the age of Coal-plants was preceded by the occurrence 

 of related plants far back in the Devonian. The age of Mammals 

 was foreshadowed by the appearance of mammals long before, in the 

 course of the Reptilian age. And the age of Reptiles was prophesied 

 in types that lived in the earlier Carboniferous age. Such is the 

 system in all history. Nature has no sympathy with the art which 

 runs up walls to divide off her open fields. 



But the question may arise, whether a geological age is not, after 

 all, strongly marked off in the rocks. Rocks are but the moving 

 sands or the accumulations of dead relics of the age they represent, 

 and are local phenomena, as already explained. Each continent has 

 its special history as regards rock-making ; and it is only through the 

 fossils in the rocks that the special histories can be combined into a 

 general system. The movements which have disturbed one continent 

 have not affected in precisely the same manner the rest, although 

 there has sometimes been a general parallelism in the changes of 

 level ; and hence there are breaks in the succession of rocks on one 

 continent, or part of a continent, that have no representatives on an- 

 other. 



When an age can be proved, through careful study, to have been 

 closed by a catastrophe or a transition which was universal in its 

 effects, the event is accepted as a grand and striking one in geological 

 history. But the proof should be obtained, before the universality is 

 assumed. Hence the conclusion, — 



Fourthly. The grander subdivisions or ages in geological history, 

 based on organic progress, should be laid down independently of the 



