PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 559 



formations of Van Hise and Leith in the eastern part of the Gogebic 

 district. Between the Keweenawan and this unconformity, Allen 

 fiinds a series of clastic sediments which are neither Keweenawan nor 

 Upper Huronian. Allen decides that the Upper Huronian of 

 Van Hise and Leith is to be correlated with the Middle Huronian 

 of the Marquette district. On much less convincing evidence 

 than in the Gogebic district he also concludes that a Middle 

 Huronian is found in the Menominee district. Following these 

 studies, a revised correlation of the Lake Superior region is offered 

 by Allen in which all the important iron formations of the region 

 are designated as Middle Huronian. 



Allen has made important studies of the region between the 

 Penokee and Iron River districts, but owing to the drift cover of 

 the area he has not obtained the facts for a satisfactory correlation 

 of the Iron River with the Penokee and Marquette districts. 

 He has also studied the Gwinn district and the eastern extension 

 of the Menominee district. 



R. C. Allen' beheves that the pre-Cambrian rocks of the Gwinn 

 district located about sixteen miles south of Marquette, Michigan, 

 comprise two unconformable series which he correlates with the 

 Upper and Middle Huronian respectively. The succession accord- 

 ing to Allen is shown on page 560. 



The graywacke and conglomerate near the middle of the sedi- 

 mentary succession constitutes the evidence on which Allen bases 

 his conclusion for unconformity in the Huronian system. The 

 conglomerate contains fragments which resemble the underlying 

 sediments including iron ore. The two unconformable series appear 

 to be structurally concordant however. 



His correlation of the lower series with the Middle Huronian 

 is based on the fact that the Lower Huronian is not known to 

 contain iron formation of the type in the Gwinn district. 



Allen and Barrett^ believe that the acid mica schists of Wolf 

 Lake are the metamorphosed equivalent of the Paint slates of 



^ R. C. Allen, "Correlation and Structure of the Pre-Cambrian Rocks of the 

 Gwinn Iron Bearing District of Michigan," Jour. GeoL, Vol. XXII, No. 6 (1914); 

 also in Mich. Geol. Surv. Pub. 18 (1915), Geol. Ser. 15, pp. 161-64. 



^ R. C. x\llen and L. P. Barrett, "The Paint Slate and the Wolf Lake Granite, 

 Gneiss and Schist," Mich. Geol. Surv. Pub. 18 (1915), Geol. Ser. 15, pp. 131-39. 



