GEOLOGICAL CHRONOLOGY 349 



Despite these limitations we find that, speaking broadly, the 

 order of succession in the appearance and extinction of the great 

 groups of fossils is much the same for all parts of the earth, and we 

 may confidently assume that the grander divisions of geological 

 time are of world-wide significance. 



It will now be easy to understand why the fossils in two groups 

 of unconformable strata are generally so radically different. It is 

 because of the long lapse of unrecorded time at that point, during 

 which organic progress continued ; when deposition was resumed, 

 the animals and plants were all new, and so the change is abrupt. 

 If one is reading a book from which a dozen chapters have been 

 torn out, the change of subject will appear violently abrupt ; to 

 bridge over the gap one must find another copy of the book. 

 Likewise, to fill up the gap of a great unconformity, we must go 

 to some region where deposition went on uninterruptedly, and 

 there we may trace the gradual and steady change in the fossils. 



A geological chronology is constructed by carefully determining, 

 first of all, the order of superposition of the stratified rocks, and 

 next by learning the fossils characteristic of each group of strata. 

 The history is recorded partly in the nature and structure of the 

 rocks, partly in the fossils, and partly in the topographical forms of 

 the land and the courses of the streams. By combining these 

 different lines of evidence, local histories are constructed for each 

 region, until from these the story of the whole continent may be 

 compiled. The comparative study of the fossils then gives the 

 clue for uniting the history of the different continents into the 

 history of the earth. Much remains to be done before this great 

 task can be accomplished, but already we have an outline of the 

 scheme which future investigations may fill up. 



It necessarily follows from the way in which sedimentary rocks 

 are formed, and the local nature of upheavals and depressions of 

 land, that in no single locality can the entire series of strata be 

 observed, and that each region can display but a certain propor- 

 tion of the whole record. The different parts of our continent 

 are of vastly different geological dates, and even the same area 

 may have been many times a land surface, and as often a sea- 



