T. C. Chamherlin — Diversity of the Glacial Period. 187 



that " There is no foreign material in the cuts, at least, 

 Professor A. A. Wright and I could not find any"; (p. 307) 

 and a second is that drift from the Medina sandstone is absent 

 from these localities, and that its absence is decisive. 



1. The absence of any single lithological element is not 

 demonstrative of the absence of drift of glacial origin. Only 

 a short distance to the north, the whole group of crystalline 

 erratics is reported by Professor White to be absent from a 

 large part of Susquehanna and Wayne counties." No one 

 questions the glaciation of that region because of their ab- 

 sence. Most experienced glacialists I think must have en- 

 countered similar instances of the local absence of a particular 

 rock constituent. Such apparent absences over considerable 

 areas have been encountered by Mr. Buell in a specially care- 

 ful tracing of bowlder trains from the isolated crystalline 

 knobs of Wisconsin, and for a time such absence seemed to 

 indicate a limitation of that particular kind of drift ; but 

 further search has shown that beyond this barren area the par- 

 ticular erratics reappear and extend onward for considerable 

 distances. 



2. It so happens that Mr. Kummel of the New Jersey 

 geological survey, without a knowledge that the localities were 

 the subjects, or were likely to be the subjects of special ques- 

 tion, made collections of. rock specimens at both High Bridge 

 and Pattenburg, simply with a view to rendering them as com- 

 plete and representative as possible. Upon request, these col- 

 lections have been sent to Chicago and examined, and Professor 

 Smock has also kindly given his opinion of them. Referring 

 to a group of specimens belonging to the collection, Professor 

 Smock expresses the opinion that " they belong to or have 

 come from the ledges of Cambrian, Green Pond Mountain 

 and Potsdam, in New Jersey and New York. Of course it 

 would be possible to duplicate the lot in the Green Pond 

 Mountain range in the Kittatinny Mountain series, and in the 

 Potsdam." These all lie at distant points. Among the speci- 

 mens are pebbles of sandstone so closely resembling the 

 Medina sandstone that only an expert familiar with the special 

 character of the formation in New Jersey could give an 

 opinion of any value at all. Professor Smock, after a careful 

 examination, does not feel justified in giving a positive opinion. 

 In the presence of these specimens, the alleged absence stands 

 upon a very slender basis. 



The collections contain specimens of gneiss, sandstone of at 

 least two distinct types, quartzite, shale of two kinds, and 

 chert. These appear to represent certainly five different 



* Sec. Geol. Sur. Perm., G 5, 1880, p. 26. 



Am. Jour. Sci. — Third Series, Vol. XLV, No. 261. — March, 1893. 

 14 



