586 HENRY SHALER WILLIAMS 



too, in the fossil characters is not so marked as to be at this time available. . . . 

 What has caused geology to advance with rapid strides has been a knowledge 

 of fossils, and when those of the upper part of the Erie division shall have been 

 fully examined, and the kind determined which are limited to a group, and those 

 which are not, then the difficulties will be at an end. (P' 172.) 



Vanuxem in 1842 may have estimated too highly the results to be 

 attained by a study of the fossils, but of the nature of the results he 

 had a clear conception. 



Neither of the sets of rules above referred to has given an intima- 

 tion of the way by which we are to distinguish the size (in thickness 

 of strata) of the geological unit, either of classification or for mapping 

 purposes. How thick may a "system" or an etage of Dewalque's 

 report be? or. How may a "formation" of the United States rules 

 be distinguished from a "member" or a "group"? The raising 

 of these questions will doubtless call forth scarcely two repHes ahke. 

 In new work as in the old, the individual is left to draw the limits 

 or boundaries of his "formation" as he will. The literature indicates 

 that usage has been as diverse as is the modern practice. 



The fact remains that it is all-important to discover if there may 

 be some means of discriminating between the major and the minor 

 divisions of the geological column. 



The taxonomic rank 0} the subdivisions 0} the first New York 

 survey. — The specific problem now before us is regarding the rank 

 to be assigned the subdivisions originally recognized by Vanuxem 

 in the Ithaca section. 



The Portage or Nunda group. — Vanuxem described the first 

 subdivision above the Genesee (no. 25) as the Portage or Nunda 

 group. It is evident that it is only by correlation that this name 

 is applicable to a part of the Ithaca section. The typical section 

 of the Portage or Nunda group is found in the Genesee valley and 

 is defined in Hall's Report of the Fourth District. In that region 

 the "group" is composed of three subdivisions, viz., Cashaqua 

 shales, Gardeau shale and flagstone, and Portage sandstones. It 

 is fair to assume that the taxonomic rank of these subdivisions be 

 tested in the original section, and that the application of the terms 

 to the section at Ithaca rests upon correctly correlating its rocks 

 with those of the original section in the Genesee valley. 



