392 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



The Trenton is the third name that is to be used with a definite 

 stratigraphic value bej^ond the confines of New York and Ontario. In 

 most areas it is readily divisible into members or distinct formations, 

 and its time value is about the same as that of the Black Eiver group. 

 In the scheme of classification that has been prepared for Part III of 

 this work^ both of these terms are given the rank of groups; but this 

 technical rank does not preclude the use of the names in the formational 

 sense, especially in areas where it is inexpedient to distinguish thin strati - 

 grapliic units. Together the two groups constitute the Mohawkian series. 



The Clinton finally is another of the old terms ranking with the 

 Black Elver and Trenton that is to be used in a similar manner. It being 

 now conceded that the Eochester shale zone is included in and forms the 

 top division of the typical Clinton, and as the part beneath the Eochester 

 is locally divisible into several lithologically and faunally distinct mem- 

 bers or formations, the composite Clinton unit manifestly has become a 

 group. It is so regarded in my proposed classification, in which it con- 

 stitutes the lower of two groups into which the Niagaran series is 

 divided. However, on grounds of expediency and convenience in map- 

 ping, and because in its eastern development the subdivisions are, as a 

 rule, not easily distinguished, the Clinton is viewed as a single strati- 

 graphic unit in the long belt extending southward in the Appalachian 

 Valley from Clinton, New York, to central Alabama. 



RELATION OF FORMATION NAMES TO TIME TERMS 



Following the recommendation of the International Congress of Geolo- 

 gists, geological time is generally classified into eras, periods, epochs, 

 stages, and ages. The stratigraphic units are similarly arranged into sys- 

 tems, series, groups, and formations, four classes theoretically correspond- 

 ing to those of the time scale below the rank of eras. But in general prac- 

 tice the stratigraphic units termed "formations" often bear no definite 

 relations to the time units of the grade of "ages." In fact, the units of 

 the lowest and next higher ranks in either scale are often far from coordi- 

 nate. This is unavoidable and remediable only with time, being due to 

 unequal exploitation of different parts of the stratigraphic column. But 

 can we not do something now to improve matters ? Must all the units 

 of a given map be treated as though coordinate, even if one includes 

 thick beds referable to two or more periods and another is but a simple 

 lithologic unit of relatively insignificant time value? Why not call one 

 the Clinton group, another the Helderbergian series? Such qualifica- 

 tions would immediately convey desirable information and indicate that 

 these formations are taxonomically more important than is the Tusca- 



