ON GEOLOGIC TIME-DIVISIONS 349 



masses also. As the stages are based largely upon lithological 

 characters and receive local geographic names, the latter are 

 followed by such words as limestone, shale, granite, etc., thus 

 doing away with the technical title altogether. In general the 

 word formation seems to be best retained for use in somewhat 

 doubtful cases, where the exact taxonomic rank is questionable, 

 but believed to be about of the fourth order ; while group refers 

 to any of the greater orders. 



The time equivalent of the stage seems best expressed by 

 the word episode. The word Time is also appropriate, and it 

 more exactly corresponds with the historical usage to represent 

 a generation. 



The zone is a useful subdivision of the smallest unit usually 

 recognized in this country, The name of its time equivalent is 

 Hemera, proposed by Buckland. The zonal classification of 

 the Ammonite-bearing beds of the Jurassic is an example. 



Assuming the ultimate aim of every scheme of geological 

 chronology to be to provide a means of paralleling stratigraphic 

 successions more or less widely separated geographically, a 

 practical question arises as to how far a general classification is 

 applicable to a given region, and how far the local plan is 

 capable of being expanded. 



While the double geological scale is theoretically every- 

 where balanced, in practice the time element is given precedence 

 at the more general end of the scheme, and the rock element at 

 the more specific or local extremity. In the present state of 

 our knowledge general correlation farther than series is beset 

 by many and grave difficulties, and it is doubtful whether it is 

 feasible to extend it beyond. 



So far as concerns the first two orders enumerated in the 

 plan already given, it seems desirable, for the present, to retain 

 the names generally applied to the different "groups," even 

 though they are largely European in origin and are not exactly 

 expressive of the real conditions in North America. They are 

 so thoroughly part and parcel of our literature that it would be 

 revolutionary to supplant them. It is better to modify their 



