CORRELATION AND STRUCTURE OF THE PRE-CAM- 

 BRIAN FORMATIONS OF THE GWINN IRON- 

 BEARING DISTRICT OF MICHIGAN 



R. C. ALLEN 



Director, Michigan Geological Survey 



Published information regarding the geology of the Gwinn dis- 

 trict is very meager. In 1873, Major J. B. Brooks published' a 

 brief description of the locality now occupied by the Princeton and 

 Stegmiller mines, then known as the S. C. Smith mine, in sections 

 17, 18, and 20, T. 45, R. 25. In speaking of the occurrence of iron 

 ore there he says: "The geographical position is less remarkable 

 than what might be called its geological isolation, for it appears to 

 be in a small patch of Huronian rocks, in the midst of a great area 

 of barren territory, underlain by the Laurentian and Silurian sys- 

 tems." Brooks observed the black slate adjacent to the ore on the 

 northeast in sections 17 and 18 and in "section 20, west of the 

 river, a talcky schist, holding grains of quartz," but was unable to 

 determine the stratigraphic relation of these rocks to the iron 

 formation. 



About ten years later this locality was again examined by 

 Dr. Carl Rominger,^ who writes as follows: "The Cheshire mine, 

 formerly known as the S. C. Smith mine .... is working a strip 

 of slaty and quartzose rock beds, known to extend along the valley 

 of the Escanaba River for a distance of nine miles from the north- 

 west corner of section 19, T. 46, R. 26, to the center of T. 45, R. 25." 

 Rominger describes the rocks shown in the mining pits in sections 

 18 and 20, T. 45, R. 25, in considerable detail. He recognizes an 

 iron formation underlain and overlain by slate. Owing to his 

 misunderstanding of the structure his succession is reversed. 



In 191 1 the United States Geological Survey published a brief 

 account of the geology of the Gwinn (Swanzy) district by C. R. 



^ Michigan Geological Survey, I, 150-51. 

 == Ibid., V, (1894), Part I, pp. 70-73. 



560 



