PART III. 



HISTORICAL GEOLOGY. 



GENERAL DIVISIONS IN THE HISTORY. 



1. Nature of subdivisions in history. — The methods of ascertain- 

 ing the true succession or chronological order of the rocks have been 

 explained on pages 101 to 107. Some further explanations are neces- 

 sary, by way of introduction to the survey of geological history. 



What are subdivisions in history ? — Many persons, in their study of 

 geology, expect to find strongly-drawn lines between the ages, 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 

 beginning 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 development into ages with bold confines are fruitless. We 

 may trace out the culminant phases of different periods in that pro- 

 gress, and call each culmination the centre of a separate period. But 

 the germ of the period was long working onward in preceding time, 

 before it finally came to its full clevelopement 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 fundamental 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 apparent. 

 The age of Mammals, the age of Reptiles, and the age of Coal-plants 

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



