340 A SYMPOSIUM 



with the ordinary meaning of the word, — an aggregate of 

 individuals (not an aggregate of aggregates). 

 (7.) No. 



(7, 8.) I like Mississippian as the title of an American sub- 

 division of the Carboniferous period. There is need of a com- 

 plementary, coordinate, American, geographic name (or names). 

 (9.) No. 

 (14.) I prefer : 

 Eon - System (or Series). 



Period - Series (or System). 



Epoch - - - Group. 



Stage (or Age) - Formation. 



(Comment on 5, 8, 10, 13.) The adjectives of space relation, 

 Lower and Upper, should not appear in a time scheme. The 

 prefixes Eo-, Meso- and Neo- (proposed for a somewhat different 

 use by Williams) seem appropriate for the indication of indefi- 

 nite portions of any time unit. For definite parts separate geo- 

 graphic names are preferable. G. K. Gilbert. 



CONTRIBUTION BY WM. BULLOCK CLARK. 



I think that the questions, which you have raised regarding 

 the use of terms in geological classification, are most timely. 

 If a discussion of the subject can aid in bringing about some 

 unanimity in the employment of these terms on the part of 

 geologists, you will have performed a great service. 



I am inclined to take the position that, from the very nature 

 of the case, a universal system of stratigraphic equivalents can- 

 not be employed for the chronologic terms. The chronologic 

 divisions, as we all recognize, are at best highly artificial, while 

 the stratigraphic divisions are natural and definitely determi- 

 nable units. The term " formation," for example, has come to be 

 rather widely used to embrace deposits formed under approxi- 

 mately similar conditions whatever the time element involved, 

 and may or may not be separated from overlying or underlying 

 formations by an unconformity. 



Accepting the chronologic terms which you have adopted, 



