DISCRIMINATION OF TIME-VALUES IN GEOLOGY 583 



them ; and the fossils should be used (as now their use is scien- 

 tifically most valuable) in establishing correlations, not in fur- 

 nishing definitions. Sixthly, the selection of the grander time 

 divisions should be made to conform, so far as found practicable, 

 with the standard divisions of the formation scale ; but their 

 discrimination should be based strictly upon fossil evidence and 

 not upon the lithologic or stratigraphic characters of formations. 

 Seventhly, in forming the time-scale the continuance of the 

 fauna in its integrity, as marked by the dominance of its 

 characteristic species, will constitute a more satisfactory basis 

 of discrimination than the species alone. Slight differences in 

 the aggregate of species found in the successive strata of rocks 

 is quite consistent with the continuance of the fauna in its integ- 

 rity, and to distinguish the faiuia as a whole from its local and 

 temporary expression the latter may appropriately be called a 

 faunule. 



In constructing a time-scale on the basis of the bionic values 

 of fossils, it is practicable to give to the categories now in use 

 greater precision and scientific definition. If geologists chose 

 to adopt the following terms in the restricted sense designated, 

 such a set of terms would prove of great value. 



TERMS OF THE BIONIC TIME SCALE. 



Chron. — An indefinite division of geological time. 



Geockron. — Tlie time equivalent of a formation. 



Biockron. — The time equivalent of a fauna or flora. 



Heinera. — The technical name for a monobiochron, indicated by the 

 preservation of the individual characteristics of all the species of a local 

 faunule, as shown by the association in the rocks of the same species, in the 

 same relative abundance, size, and vigor. An example is the hemera of 

 Rhynchonella {Hypothy7'is) ctiboides. 



Epoch. — The name of a dibiochron, indicating the time equivalent of the 

 endurance of a particular species and of the integrity of the fauna of which it 

 is the dominant characteristic. An example is the Tropidoleptus carinatus 

 epoch, which corresponds closely to the limits of the Hamilton formation of 

 eastern New York. 



Period. — -May be defined as a tribiochron. This is the time equivalent of 

 the continuance of a genus. An example is the Paradoxides period, which 

 corresponds to the Acadian formation of the Cambrian system. 



