ON GEOLOGIC TIME-DIVISIONS 34 1 



and which, I think, cannot be improved upon, certainly to the 

 third division — viz., era, period, epoch — -it may be possible to 

 find the formation as the equivalent of a portion or the whole 

 of any one of these time divisions, excepting, perhaps, the era. 

 To attempt to restrict it, therefore, in all instances to any chron- 

 ologic division, large or small, would seem to me unwise. 



Furthermore, I think that a different series of names should 

 be applied to the formations and their subdivisions than to the 

 time units. I should speak of the Palaeozoic era or time, the 

 Cambrian period, and the Upper Cambrian, or better, the Neo- 

 Cambrian epoch, but of the Potsdam formation or the Shenan- 

 doah formation, the latter representing portions of the Lower 

 Silurian as well as Upper Cambrian, and affording a good exam- 

 ple of the formational unit. I prefer the prefixes Eo-, Meso- and 

 Neo- to designate the epochs, as proposed by Williams. I think 

 the term Stage more applicable to a division of a formation, 

 whether characterized by a distinct fauna or not, than to a time 

 unit. 



In reply to your questions seven and eight regarding the later 

 divisions of the Palaeozoic, I should employ the chronologic 

 terms Carboniferous and Permian, the former divided into Upper 

 and Lower, or Upper, Middle and Lower Carboniferous, as the 

 case might be. To be consistent the terms Eo-, Meso- and Neo- 

 Carboniferous should be used. The Upper Carboniferous may 

 be represented by Coal Measures made up of one or more forma- 

 tions ; the Lower Carboniferous may be represented as in the 

 central United States, by the Mississippian, or, as I should prefer, 

 the Mississippi Group, made up of various formations. I should 

 personally object to the use of the term Mississippian in a 

 chronologic sense, unless the period term Carboniferous was to 

 be permanently divided and the resultant divisions raised to the 

 period rank. The reasons for such change, however, do not 

 seem to me to be sufficiently strong. I think the widely 

 extended difference in facies represented in the Upper and 

 Lower Carboniferous tends greatly to accentuate the two divi- 

 sions of this period. 



