THE DEVONIAN SECTION OF ITHACA, N. Y. 585 



(the formations) are subdivided on the basis of differences in 

 composition locally expressed. 



Systems the European units 0} classification. — In Europe, prac- 

 tically, the "systems" are the primary units found already defined, 

 named and in common use; and subdivisions of the systems are 

 called respectively, according to their order of rank, series, Stages, 

 and assises (these are the French terms). 



Terms "group'' and "series.'" — The terms "group" and "series" 

 are also adopted in the United States Geological Survey rules, but 

 they are there used for aggregates of formations, subordinate to 

 systems and determined by local structure rather than simple corre- 

 lation. Rule 20 sets forth this practice. 



Within the system smaller aggregates of formations may be recognized which 

 shall be called "series," and these may be divided into subordinate groups of 

 formations .... These minor aggregates should be formed so as to express 

 the natural relations of the formations of the particular province rather than to 

 conform w^ith divisions recognized elsevi^here. 



The fundamental difference is exhibited in this rule 20: "Sys- 

 tems" are subdivided in Europe into series, etages, assises; in Amer- 

 ica "formations" are aggregated into "groups," "series," and 

 "systems." 



In practice American geologists, unless actually working on 

 the survey under the rules of the United States Geological Survey, 

 are more accustomed to adopt the principles of the European rules 

 than those of the United States Geological Survey. This result is 

 perhaps because the European system lends itself better to the 

 use of textbooks and colleges. 



Uncertainty as to the definition of "formation.'" — In the present 

 revision particular attention has been given to meeting the require- 

 ments of the rules of the United States Geological Survey. To do 

 so, one of the first, and certainly an important, task required of the 

 investigator is to determine which of the subdivisions of the section 

 proposed by Vanuxem is a "formation" under the rules. Vanuxem 

 reahzed the nature of the problem when he wrote : 



There are difficulties in the beginning of most, if not all, subjects. Those 

 in the three groups under consideration arise from the little difference in their 

 mineral characters, which, for distant points, cannot be relied upon. The change, 



