488 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



In Prof. T. C. Chamberlin's Report, prepared from the notes of the 

 late Moses Strong, it is stated of the same locality : — 



" This locality presents the most clear and nnequivocal evidence that the 

 Copper-hearing series is much older than the Potsdam sandstone of our state, 

 so much older, indeed, that there was time for the very extensive wearing 

 down of the former hefore the latter was deposited." (Geol. of Wise, 1880, 

 III., pp. 416-419.) 



Professor Irving later carried his work into the Michigan region, and 

 thus states the conclusions at which he ai'rived : — 



" The series is older than the Camhrian and younger than the Huronian — 

 the separation from the former heing by an intervening disturbance and ero- 

 sion, and from the latter by an intervening erosion, and possibly also by an 

 mtervening folding and alteration." 



He further found the acidic rocks, from which most of the Keweenaw 

 conglomerates were derived, in place, and adopted the published view of 

 Dr. Wadsworth {ante, pp. 113-122), that these rocks are old rhyolites 

 and trachytes. (Report of the Director of the United States Geological 

 Survey [2d Annual], 1882, pp. xxxi.-xxxiv.) Irving also adopted the 

 theory of a synclinal structure of the region, first announced by Foster 

 and Whitney. 



Mr. A. R. Selwyn later claimed that he had not expressed any opinion 

 regarding the age of the trap and sandstone of Lake Superior, forgetting 

 the previous expression of his views that they were Huronian (see ante, 

 pp. 106, 108), but gives as the result of his studies in 1882 the view 

 that 



" they occupy the geological interval elsewhere filled by those divisions of the 

 great lower paleozoic system which underlie the Trenton group. Various 

 considerations point to the Potsdam and Primordial Silurian (Lower Cambrian) 



as their nearest equivalents They are essentially volcanic, sub-aerial, 



and sub-aqueous formations, and in every sense analogous to the wide-spread 

 tertiary volcanic rocks of Australia and other regions. The only diff'erences 

 are their greater antiquity and the consequent greater changes and modifica- 

 tions they have undergone through the operation of long-continued metamor- 

 phic agencies, disturbance, and denudation ; though these changes are far less 

 than those which rocks of the same age, and to some extent similar origin, 

 have undergone in eastern America and in Britain." (Science, 1883, I., pp. 

 11, 221.) 



It is obvious that these views are identical with those published by 

 Foster and Whitney in 1850. {Ante, pp. 76-131 ; see also Science, 

 1883, L, p. 39.) 



