2 THE MARQUETTE IROX-BEAKIXG DISTRICT. 



A preliminary report lias been published by us in the Fitteeiith Annual 

 Report of the United States Geological Survey. 



The present report is based upon a detailed examination of the entire 

 Algonkian area from Lake Superior to Lake Michiganune. Topographic 

 maps of a part of the district, made by the United States Geological Survey, 

 have been supplemented in critical areas by large-scale plane-table sheets. 

 Practically all outcro]js have been accurately mapped on a large scale. 

 In the mining part of the ai'ea advantage has been taken of underground 

 workings and borings. Owing to the detailed character of this work, 

 combined with the advance of geological knowledge in the past twenty 

 years, it is now possible to present a much more satisfactory account of tlie 

 structure of the district than has yet been given. 



Notwithstanding the fact that mining has been done for many years 

 in the district, it is little traveled away from the roads. The timber has 

 been cut off for iron smelting. Where the cut has been comparatively recent 

 the fires have ran, and there is now a tangle of fallen timber and briars 

 and bushes. Where the cut is older there is a thick second growth, 20 to 

 50 feet high. The area is therefore much more difficult to penetrate than 

 was the primeval forest. Moreover, the bushes are an eftectual bar to 

 extended vision, except from high, rocky points. While the disti-ict is not 

 mountainous, in detail much of it is exceedingly rough, so that in travers- 

 ing parts of it one is nearly always ascending or descending a steep slope. 

 Other parts are covered by a mantle of glacial deposits, through which the 

 rocks rarely protrude. Because of the irregularity of the topograjjliy and 

 the difficulty of seeing, it has been impossible to base locations on the 

 ordinary topographic maps. For the larger part of tlie district locating 

 was done either with the aid of the plane-table or l)y pacing from section 

 corners and cjuarter posts. 



The rocks of the district comprise three series, separated by uncon- 

 formities. Each of these series consists of several formations. The 

 transgression of the sea did not occur o^'er the entire district even 

 approximately at the same time, so that in parts of it the succession is not 

 complete, and in other parts the succession is incomplete because of inter- 

 vening erosion. Finally, the district has been folded in a complicated 

 fashion in two directions, with resulting profound metamorphism. 



