564 R. C. ALLEN 



accompanying map that information is entirely wanting in some 

 parts of the syncKnorium and in other parts is insufficient for accu- 

 rate mapping. Only a few of the many faults, which certainly 

 occur, particularly in the north end of the district, have been 

 mapped and the exact location and character of even those is not 

 apparent. 



The lithology of the various formations will be considered only 

 so far as essential to an understanding of the succession and the 

 correlations, but the discussion necessarily will be more in detail 

 than the account published in Monograph j2, to which reference 

 has been made. 



Preliminary to the statement of the geology, there is given in 

 parallel columns for comparison the succession and correlation of 

 the United States Geological Survey and of the writer. 



LOCATION AND TOPOGRAPHY, ETC. 



The Gwinn synclinorium occupies an area about six miles long 

 and from one to two miles wide, mainly in T. 45 N., R. 25 W., but 

 extending a short distance into T. 44 N., R. 25 W. The trend of 

 the major structure is about N. 45° W. or almost exactly parallel 

 to the Repubhc trough, the southern end of which is 22 miles west 

 and 6 miles north of the north end of the Gwinn fold. Gwinn, the 

 principal village, is 16 miles south of the city of Marquette. 



The southeast three-fifths of the Gwinn fold is buried beneath a 

 featureless and almost flat sand plain which extends north and east 

 to the hills of the Marquette range. In the opposite direction the 

 surface is broken and hilly with occasional rock exposures. Granite 

 hills encircle the northwest and north sides of the synclinorium. 

 The district is drained by the Escanaba River, which follows the 

 northeast side of the trough to Gwinn and then turns south across 

 the sand plains. On the plains the water table is within a few feet 

 of the surface and the ore bodies are deeply buried under water- 

 saturated sand and gravel, a condition which is a serious menace 

 to mining operations. 



The first shipment of ore was made in 1872 from the Cheshire 

 mine, now known as the Princeton No. i pit. About 1902 the 

 Cleveland Cliffs Iron Co. purchased the Princeton (Swanzy, 



