112 THE MAEQUETTE IRON BEARIIS' G DISTRICT. 



Tlie Huroniiiu and the Keweenawan series together represent a great 

 interval between Archean and Cambrian times. They may be inckided in 

 one group, comprehending the fragmental series between the Archean 

 crystallines and the Cambrian fragmentals. For this group the author 

 proposes the designation Agnotozoic, because there are here and there traces 

 of life in some of the rocks belonging to it, but the nature of this life is 

 unknown. For the Marquette area, then, as well as for the remainder of 

 the Lake Superior region, he gives the following succession: 



(1) The great Basement Complex, of crystalline schists, gneiss, and 

 granite, as to whose further divisil)ilitv or nondivisiliility no opinion is now 

 expressed. Unc( mformity . 



(2) The Huronian series, mainlv of detrital rocks. Uncontbrniitv. 



(3) The Keweenawan series, of interleaved detrital and eruptive beds. 

 Unconformity. (Absent from the Marquette range proper.) 



(4) The Potsdam or Upper Cambrian sandstone. 



Irving, R. 1). On the i-lassitication of the early Cambrian and pre-Cauibrian 

 formations. A brief discussion of principles, illustrated by examples drawn mainly 

 from the Lake Superior region. Seventh Ann. Rept. U. S. Geol. Survey, for 18S5-SG, 

 Washington, ISSS, pages 365-45-4. 



In the following year the same author pul)lished a paper which has 

 already become one of the classics of geological literature. In it he enun- 

 ciates and discusses the principles that should determine the classification 

 of nonfossiliferous rock series, and illustrates their application by appeal to 

 the pre-Cambrian formations in the Lake Superior region. 



After explaining in full the significance of unconformities and basal 

 conglomerates and of lithological difterences in establishing time relations 

 between contiguous formations, the author cites examples from the JMarquette 

 district, among others, to emphasize his points. 



Time gaps are shown to have existed between the deposition of the 

 lowermost layers of the Potsdam sandstone and the formation of the under- 

 lying granites, and l)etween the })roducti(>n of the former rocks and of 

 the fragmental beds usually classed as Huronian. A picture of tiie uncon- 



