384 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



accurate correlations with the pre-Pennsylvanian "Carboniferous" lime- 

 stones of the Cordilleran troughs shall have been made it can not be 

 said to what extent the gaps in the known record of the Waverlyan are 

 bridged over in such areas of relatively continuous deposition. 



The 2,500 feet of Tennessean limestone are made up from the great 

 thickness of Newman limestone and the estimated limestone values of 

 the calcareous Pennington shale and of the more clastic Princeton and 

 Bluestone formations in the Appalachian Valley north and northwest of 

 Bristol, Virginia. The estimated limestone value of the Chester group of 

 formations in southwestern Virginia is fully corroborated by correlation of 

 the successive beds with those in the section of Monte Sana near Hunts- 

 ville, Alabama. That the estimate probably falls short of the truth is in- 

 dicated by the presence of considerable beds in the Appalachian Valley 

 that are absent in Monte Sana, western Kentucky, and southern Illinois. 



The estimated thickness of the Pennsylvanian is taken from published 

 accounts of sections in Nevada, New Mexico, and western Texas, where 

 much the greater part of this system is represented by limestone deposits. 



Suggestions concerning Nomenclature of stratigraphic Units 



GENERAL DISCUSSION 



Effect on nomenclature of shifting or 7nodification of boundaries of 

 stratigraphic units. — With any considerable shifting or modification 

 of the boundaries of stratigraphic units the matter of nomenclature be- 

 comes a serious problem. For instance, if in the revision of a system it 

 loses certain beds at either its top or bottom or from both extremities : 

 shall we continue to apply the same name to the residium or should it be 

 christened anew? So long as it retains the main and most characteristic 

 part of the system as it was originally defined, or, in some instances, as 

 it became generally known, it seems to me desirable to retain the old 

 name. The revised Silurian, from which the greater part of the old 

 "Lower Helderberg group" is removed and to the base of which it is now 

 proposed to add the Eichmond — changes that do not materially alter the 

 prevailing conception of the system — illustrates these cases. Rather more 

 diflScult are those instances, like the Ordovician, in which the generally 

 accepted estimate is cut into two almost equal parts. But even in this 

 case, may not tlie desire to preserve the old name justify its restricted 

 application to the more typical part of the divided system? IJnques- 

 tionabl}^, the Glenkiln and Hartfell shales and the Bala, in Great Britain, 

 and the beds between the base of the Chazy and the top of the Maysville 

 group, in America, are more characteristically associated in the minds of 

 geologists with Murchison's Lower Silurian, or with its substitute Ordo- 



