PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 563 



Allen and Barrett' find that a group of dominantly clastic 

 sediments overlie with marked unconformity the Upper Huronian 

 of Van Hise and Leith in the Gogebic district. They also find 

 that a granite intrudes the Upper Huronian. Their threefold 

 division of the rocks between the Keweenawan and the Archean 

 of the Gogebic, they claim, identifies this succession with the 

 three Huronian divisions of the Marquette district. They desig- 

 nate as Middle Huronian, the Upper Huronian of Van Hise and 

 Leith in the Gogebic district and extend this change to the corre- 

 lation by tliese authors to every other district of the Lake Superior 

 region, outside of the Marquette district. The revised correlation 

 table which Allen and Barrett offer places every important Lake 

 Superior iron formation excepting that of the Vermilion district 

 in the Middle Huronian. 



In 191 9, Allen^ extended to the Menominee district the three- 

 fold classification of the Huronian which he had found applicable 

 to the Gogebic district of northern Michigan. His revised correla- 

 tion of the Huronian of the Menominee district is shown on page 564. 



In previous correlations of the Menominee, notably that of 

 Bayley in Monograph 46 of the U.S. Geological Survey, the Middle 

 and Upper Huronian of Allen were regarded as a conformable suc- 

 cession designated Upper Huronian. Van Hise and Leith, in 191 1, 

 recognized a Middle Huronian quartzite, but later Leith is said 

 to have given up this revision. Allen bases his separation of the 

 Upper Huronian of Bayley on the fact that some drill holes have 

 shown what is interpreted as a basal conglomerate between the 

 Hanbury slate and the Curry iron formation. Many drill holes 

 do not show this conglomerate. He also appeals to the fact that 

 the RandvUle dolomite in different places is covered by various 

 formations ranging from the Traders iron formation to the Han- 

 bury slates. Bayley had accepted one of the alternative explana- 

 tions for this fact, viz., that the formations overlying the Randville 

 were all conformably deposited on a very uneven surface of the 



'R. C. Allen and L. P. Barrett, "Contributions to Pre-Cambrian Geology," 

 Mich. Geol. Surv. Pub. 18 (1915), Geol. Ser. 15, pp. 13-164, 12 pis., 11 figs., maps. 



' R. C. Allen, "Correlation of Formations of Huronian Group in Michigan," 

 Am. Inst. Min. and Met. Eng. (1919), No. 153, pp. 2579-94. 



