PRE-CAMBRIAN FORMATIONS OF GWINN IRON DISTRICT 569 



of the bowlders are lying free on the dump. In addition to the rocks 

 represented in the exposures in section 19 there are many bowlders 

 of granite and greenstone. 



The origin of the quartzite and slate pebbles is of great interest 

 in its bearing on the correlation of the Gwinn series. Near Little 

 Lake, about five miles east, in a range of hills on the north side of 

 section 19, T. 45, R. 24, there are numerous outcrops of quartzite, 

 quartz slate, and arkose. Van Hise and Leith considered these 

 rocks to be the base (Goodrich quartzite) of the Gwinn series which 

 we have described. In fact, their description seems to apply 

 mainly to these exposures and not to the basal member of the Gwinn 

 series as it actually exists in the Gwinn synclinorium. There is an 

 arkose and arkose conglomerate in these exposures exactly similar 

 even to the pebbles in its associated conglomerate, to the basal 

 member of the Gwinn series. This formation, however, is plainly 

 uncomformably below the quartzites and quartz slates, as proven 

 by the occurrence of a coarse conglomerate at the base of the quart- 

 zite carrying numerous bowlders of the arkose some of which are 

 as much as 2 ft. in diameter. The exposures at Little Lake are 

 not in the Gwinn synclinorium but will be described in a later paper. 

 The point is emphasized, however, that the presence of quartzite 

 and cherty, quartz-slate pebbles in the basal member of the Gwinn 

 series proves that there is at least one unconformable series of sedi- 

 ments between the Archean and the Gwinn series. The writer 

 believes that this series is the Lower Huronian as represented in the 

 Marquette district a short distance north. 



2. The lower slate. — -In the southeastern three-fifths of the dis- 

 trict a black, graphitic, and gray slate formation intervenes between 

 the basal arkose member and the iron formation. It is less gener- 

 ally present from the Stephenson mine northward, in this area 

 never exceeding a few feet in thickness, but south of the Stephenson 

 mine it varies up to above 60 ft. thick. Were it not for hthologic 

 dissimilarity this slate would be included in the basal member, but 

 inasmuch as it represents a distinct change in conditions of sedi- 

 mentation and moreover seems to maintain a definite stratigraphic 

 relation to the overlying and underlying formations, it should 

 perhaps be described as a distinct member of the series. 



