ON GEOLOGIC TIME-DIVISIONS 345 



sions prior to the Carboniferous and refrain from expressing an 

 opinion about them. 



I very much prefer the use of European terms for divisions 

 that can be correlated with tolerable exactness ; otherwise dis- 

 tinctive American terms should be used. 



I sincerely hope that you will bring some order out of what 

 has been so confusing to both teacher and student. 



S. W. Williston. 



CONTRIBUTION BY BAILEY WILLIS. 



Your inquiries of May 5, concerning the use of certain com- 

 mon terms in geology and questions of classification, were duly 

 received and have been carefully considered. In answering I 

 beg to state that I express mv personal opinion as determined 

 by experience in practical field work and in editorial work on 

 geologic maps. 



The following answers are arranged categorically, according 

 to the numbers of the questions to which they refer. 



1. Eras, Systems. — Terms to be applied respectively to the 

 grand divisions of time and the rocks representing them, as 

 determined by the most important events of biologic develop- 

 ment. 



Periods, Groups. — Arbitrary divisions respectively of time 

 and rocks within the eras and systems, designed to afford 

 means of approximate designation of the position of any geologic 

 record in the time scale. These should be applied consistently 

 the world over according to the volume of stratigraphic evidence 

 as checked by palaeontology, but it does not necessarily follow 

 that in North America they designate time divisions precisely 

 contemporaneous with those distinguished in other continents. 



Ages, Series. — Terms to be applied respectively to sub- 

 divisions of time and rocks less than period and group, but 

 including a consistent sequence of biologic or lithologic changes 

 without break. An age or series may include parts of two 

 periods or groups. 



Epochs, Formations. — Terms applied to designate the time 



