7o6 C. K. LEITH AND R. C. ALLEN 



studies have been published in a series of monographs on the most 

 important districts and a final summary covering the entire region.^ 

 In 1892 Irving and Van Hise published their completed work 

 on the Gogebic iron range. They beheved that the Huronian 

 series in the Gogebic district comprises two unconformable series, 

 the Upper and the Lower Huronian, and that the Upper Huronian 

 or iron-bearing series is probably equivalent to the Animikie of the 

 north shore of Lake Superior. In 1896 Van Hise, Bayley, and Smyth 

 issued a monograph on the Marquette iron range. They found 

 here two unconformable Huronian series, the Upper and the Lower 

 Huronian, which were correlated with the Upper and the Lower 

 Huronian of the Gogebic Range, the Negaunee iron-bearing series 

 of the Marquette Range falling in the lower Huronian. These two 

 works are the origin of the dual classification of the Huronian 

 which was gradually extended to cover all of the other districts 

 of the Lake Superior region until A. E. Seaman discovered, about 

 1902, that the "lower" Huronian of the Marquette Range is divis- 

 ible by a great unconformity at the base of the Ajibik quartzite. 

 In 1904 Seaman's discovery was formally recognized by an inter- 

 national committee of Canadian and United States geologists which 

 adopted the tripartite classification of the Huronian group .^ The 

 Negaunee iron-bearing series was separated from the Lower Huro- 

 nian to form the new Middle Huronian, but the old correlations out- 

 side the Marquette Range were preserved. In 19 13 Allen accounted 

 for a tripartite division of the Huronian in the Gwinn synclinorium 

 and correlated the Gwinn iron-bearing series with the Negaunee 



^ R. D. Irving and C. R. Van Hise, "The Penoke Iron Bearing Series of Michigan 

 and Wisconsin," Monograph 19, U.S. Geol. Survey, 1892; C. R. Van Hise and W. S. 

 Bayley, "The Marquette Iron Bearing Series of Michigan, with a chapter on the 

 Republic Trough, by H. L. Smyth," Monograph 28, U.S. Geol. Survey, 1896; C. R. 

 Van Hise, J. M. Clements, W. S. Bayley, and H. L. Smyth, "The Crystal Falls Iron 

 Bearing District of Michigan," Monograph 36, U.S. Geol. Survey, 1899; C. K. Leith, 

 "The Mesabi Iron Bearing District of Minnesota," Monograph 43, U.S. Geol. Survey, 

 1903; J- M. Clements, "The Vermilion Iron Bearing District of Minnesota," Mono- 

 graph 45, U.S. Geol. Survey, 1903; W. S. Bayley, "The Menominee Iron Bearing 

 District of Michigan," Monograph 46, U.S. Geol. Survey, 1904; C. R. Van Hise and 

 C. K. Leith, " Geology of the Lake Superior Region," Monograph §2, U.S. Geol. Survey, 

 1911. 



^Jour. Geol., XXII, No. 6 (1914). 



