STRATIGRAPHIC TAXONOMY * 581 



Under this conception a geological age or time unit of the lowest rank 

 determinable by diastrophic movements may be defined as a definite span 

 of time. Displacement of the strandline being accepted as the dominant 

 ■criterion a geological age is regarded as having closed when the marine 

 Avaters were largely or wholly withdrawn from one or more of the conti- 

 nental basins, the succeeding new age as having opened when the sea 

 again began to advance in the same or in other basins. Accuracy in the 

 •delimitation of time divisions therefore depends solely on our ability to 

 correlate exactly the stratigraphic breaks in neighboring and distant 

 basins. First we must be able to decide that the stratigraphic hiatus in 

 one case is, geologically speaking, small, in another great. Then we must 

 be able to say just how much of the hiatus is elsewhere represented by 

 ■deposit. Finally, we should know whether or not a stratigraphic break is 

 discernible at the base of the most complete depositional record known of 

 a given age. AVlien all these factors have been determined then, and only 

 then, will we have exhausted the possibilities in the way of exactitude in 

 •delineation of time units. 



Although the subject, in so far as the Ordovician in America is con- 

 cerned, is far from being exhausted, it is yet true that great progress in 

 detailed correlation has been made in the classification of the rocks of 

 this period. And what is possible in the Ordovician is certainly more 

 readily achievable in the case of the more widely accessible and as a rule 

 more highly fossiliferous younger systems. It is only the older systems, 

 which are largely confined to disturbed areas and in which fossils are less 

 generally distributed, that present greater difficulties. 



Revision of metliods as well as facts of stratigrapJdc classification. — 

 It will have been observed that my revision affects the methods as w^ell 

 as the facts of stratigraphic correlation. By method, I mean the manner 

 of determining what constitutes a geologic period or system, an epoch 

 or series, or a stage or group, and, more particularly, how the boundaries 

 of these time and stratigraphic divisions are to be drawn. Concisely 

 stated, the method followed is to divide the stratigraphic sequence at the 

 first plane beneath the introduction of a new fauna or beneath a marked 

 faunal change that exhibits evidence of diastrophic movements. If the 

 plane marks a great faunal break, providing the compared faunas in- 

 vaded from the same oceanic basin, and especially when the plane corre- 

 sponds also to a consiflera])le change in the provincial boundaries tlmt 

 had prevailed during the greater part of the preceding period, tlien i1 

 seems to me it marks the beginning of a new system. It is on such 

 grounds that the old Mississippian is divided into two systems — ihv 



