PART TV. 



HISTORICAL GEOLOGY. 



SUBDIYISIONS IlSr GEOLOGICAL HISTOEY AND METHODS OF 



CORRELATIOK 



NATURE OF SUBDIVISIONS IN THE HISTORY. 



In the study of geology, there is often an expectation to find strongly 

 drawn lines between the eras and periods, or the corresponding subdivisions 

 of the rocks ; but geological history is like human history in this respect. 

 Time is one in its course, and all progress one in plan. 



Some grand strokes there may be, — as in human history there is a begin- 

 ning in man's creation, and a new starting-point in the advent of Christ. 

 But all attempts to divide the course of progress in man's historical devel- 

 opment into periods with bold confines are fruitless. We may trace out the 

 culminant phases of different periods in that progress, and call each culmi- 

 nation the center of a separate period. But the germ of the period was long 

 working onward in preceding time, before it finally came to its full develop- 

 ment and stood forth as the characteristic of a new era of progress. It is all 

 one progress, while successive phases stand forth in that progress. 



In geological history, the earliest events were simply physical. While 

 the inorganic history was still going on (although finished in its more funda- 

 mental ideas), there was, finally, the introduction of life, — a new and great 

 step of progress. That life, beginning with the lower grades of species, 

 was expanded and elevated, through the appearance of new types, until the 

 introduction of Man. In this organic history, there are successive steps of 

 progress, or a series of culminations. As the tribes, in geological order, pass 

 before the mind, the reality of one age after another becomes strongly appar- 

 ent. The era of Mammals, the era of Reptiles, and the era of Coal-plants 

 come out to view, like mountains in the prospect, although, if the mind 

 should attempt to define precisely where the slopes of the mountain end, as 

 they pass into the plain around, it might be greatly embarrassed. 



We note here the following important principles: — 



First. The reality of an era in history is marked by the development of 

 some new idea in the system of yjrogress. . 



397 



