DUAL NOMENCLATURE. I 59 



equivalency of stratigraphical position by the structure from some 

 region in which such marks are evident. 



The Pocono and the Mauch Chunk are, as formations, but the 

 continuation of the Catskill formation upward and are of local 

 value as formation-names, but of little or no value as elements of 

 a time-scale. 



Thus the Catskill may continue to appear in the list of forma- 

 tions of New York, and the Appalachian province, where it gener- 

 ally appears as a Neodevonian formation, but it is useless to define 

 or to discuss its more exact position in the time-scale, because its 

 own time-criteria range through the whole Devonian Era, and its 

 relations to formations whose time-criteria are more exact is indefi- 

 nite and inconstant. 



If we grant the truth of the general principle here set forth, 

 it is evident that the increase of formation names can be no more 

 objectionable than the increase of the names of mountains ; and 

 that there is no more reason for applying the same name to two 

 formations which are of the same age, but differ in position and 

 structure, than there is in giving two names to mountain ridges 

 of the same range. The classification of formations into groups 

 or systems will depend upon considerations of geological structure, 

 and only secondarily upon time considerations ; and even in the 

 latter case, only because the same general geological events have 

 affected the sedimentation in a similar way for a wide extent 

 geographically. 



On the other hand, the time-scale will depend for its classifi- 

 cation upon the fossils and not upon structure. Stratigraphical 

 sequence, of course, is important in reading fossils, but only as 

 in a sentence, or on a page, the sequence of the words is impor- 

 tant. 



An organic species, or a genus, or an order lived only during 

 a particular period of time, and it is for this reason that the fossils 

 have an intrinsic time-value, but a mineral, or a rock may have 

 been formed under like conditions and present like characteristics 

 at any period of geological time. I refer of course to clastic 

 rocks, and those formed through processes of sedimentation. 



