STRATIGRAPHIC NOMENCLATURE 1109 



Where fully developed most formations include several zones. 

 Thus, in Maryland and West \'irginia, the Oriskany formation, 

 which belongs to the Oriskany stage of the Lower Devonic series 

 of eastern North America, contains at least two zones, the upper, or 

 Hipparionyx proximiis, zone, 258 feet thick, and the lower, 90 

 feet thick with Anoplothcca flabellitcs and other fossils. In some 

 cases, however, what has been considered a single formation may 

 represent an aggregation of apparently uniform lithic character, 

 of such great stratigraphic range as not only to transgress the limit 

 of a series, but even that of a system. An example of this is found 

 in the Arbuckle and Wichita sections of Indian Territory and Okla- 

 homa, where the following pre-Mississippic formations were for- 

 merly recognized : (Ulrich-i6.) 



f Upper * Woodford chert (Woodford formation) probably Post- 

 ) Devonic 



Devonic I Middle Absent 



t Lower 



C Upper 

 Siluric \ Middle 

 I Lower { 



Hunton limestone (Hunton formation) 



Sylvan shale (Sylvan formation) 



{ Upper { \ 

 Ordovicic { Middle f Viola limestone (Viola formation) 

 I Lower Simpson formation 



r Upper I Arbuckle limestone (Arbuckle formation) 

 Cambric \ Middle { > Reagan sandstone 

 L Lower Absent 



It has since been found that the Hunton formation is not a 

 unit, but represents fragments of several distinct formations sep- 

 arated by large breaks and unrepresented time intervals (13). 



Principles Governing the Naming of Formations. 



A formation may retain its name only so far as its essential 

 unity is retained, though change in lithic character does not neces- 

 sarily require a change of name. Thus, when a shaly formation 

 in one locality can be traced into, and can be shown to be the ex- 

 act depositional equivalent of, a limestone formation in another 



* The classification is by the author, and is made in harmony with the 

 present classification of the formations supposed to be included in the divisions 

 given. 



