PART III. 



HISTORICAL GEOLOGY. 



GENERAL DIVISIONS IN THE HISTORY. 



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

 taining the true succession or chronological order of the rocks 

 have been explained in \\ Y12-VL§, and in connection ( $ 126 ) a 

 brief mention is made of the grander divisions of the series. Some 

 further explanations are necessary as introductory 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 his- 

 tory 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 progress, and call each culmination the centre 

 of a separate period. But the germ of the period was long work- 

 ing onward in preceding time, before it finally came to its full 

 development and stood forth as the characteristic of a new era of 

 progress. It is the same with the development or history of an 

 individual being. There are distinct epochs and periods in the 

 history which all recognize, — the period of the embryo, of the youth, 

 of the adult. But no one thinks of marking the hour or day 

 when one ends and another begins, or of pointing to a visible 

 physical line that at any given moment was passed. It is all one 

 progress, while successive phases stand forth in that progress. 



In geological history, the earliest events were simply physical. 



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