MUSEUM OF COMPARATIVE ZOOLOGY. 105 



lithologicfxl affinities exist, that it is most natural to consider them as 

 the consecutive products of one and the same epoch, in the commence- 

 ment of wliich the just-formed strata were displaced by volcanic action, 

 which subsided toward the end and loft the last deposits undisturbed." * 



In 1878, Prof Pumpclly published a paper entitled the " Metaso- 

 matic Devolopment of the Copper-bearing Rocks of Lake Superior." f 

 This is devoted principally to a description of tlie microscopic characters 

 of these rocks and tlieir alterations, and, although we ditfcr in nomen- 

 clature, we regard it as one of the very best papers published upon mi- 

 croscopical lithology. His geological ideas remain the same, however, as 

 he states that the greater age of the Keweenaw series over the Potsdam 

 sandstone is proved by abundant evidence of non-conformability. He 

 regards them " more nearly conformable to the underlying highly tilted 

 Huronian schists. They are thus the pi'oduct of the earliest eruption 

 of basaltic rocks to which a proximately definite age can be assigned. 

 They were preceded by very extensive eruptions of acid rocks, especially 

 porpliyries. These basaltic rocks have been subjected to a wide-reaching 

 alteration, which has produced marked changes in the internal condition 

 of the beds, and has filled the fissures with a rich variety of minerals, 

 whose constituents were derived from the products of tliis alteration." 

 (L c, pp. 253, 254.) These old basaltic rocks were considered to be 

 melaphyrs and diabases. 



It is to be seen that he now adopts the views of the origin of the 

 traps held by Messrs. Foster and Whitney, and later by his assistant, 

 Marvine. The reasons for this radical change of base are not stated : 

 we have simply the assertion that the traps are basaltic overflows, made 

 as though no one had ever held a different opinion. 



Later, Dr. T. Sterry Hunt says that it seems pi-obable from our pres- 

 ent state of knowledge that the traps are of volcanic origin. He re- 

 gards them as unconformable with the Huronian, as he takes the 

 Copper-bearing rocks as the equivalent of his Taconian. This, of course, 

 requires the intercalation of his Montalban between them and the 

 Huronian. t 



Dr. J. D. Dana in his Manual of Mineralogy and Lithology,§ says 

 concerning the occurrence of the native copper with disseminated silver : 

 " This mixture of copper" and silver cannot be imitated by art, as the 



* Geological Survey of Jlichigan, 1S73-1876, III. 154. 

 t Proc. Am. Academy, XII I. 253-309. 

 } Sec. Geol. Survey Penn. E, Azoic Rocks, Part I. p. 236. 

 §• New York, 1878, p. 131. 



