20 THE MARQUETTE IRON-BKARI^'G DISTRICT. 



BuuT, Wm. a. Geological report of the survey, " with reference to miues and 

 minerals," of a district of towusliip lines in the State of Michigan, in the year 1846, 

 and tabular statement of specimens collected. Dated March 20, 1847. 31st Congress, 

 1st session, 1849-50. Senate Documents Vol. Ill, No. 1, pages 812-875. With maps, 

 oi)p()site page 880. 



The experiences of a second season in the Upper Peninsula of Michigan 

 resulted in Burt's second report. In this the author generalized to a greater 

 extent than was possible in his earlier report. The territory surveyed during 

 this second season embraced all of the "Marquette district," in addition to the 

 country north and south of it for a number of miles. The general geology 

 of all this area is briefly outlined, and the results of the 3-ear's work are 

 indicated on a map. 



The granites of the region are said by the author to pass in some 

 instances into syenitic greenstones. They are cut Ijy trap and are very 

 often gneissic in structure. In Ts. 47 N. and 48 N., Rs. 27 W., 28 W., and 

 29 W., are argillaceous slates that are the extension westward of the slates 

 observed in 1845. These slates dip " at a high angle, generally conforming 

 to the surrounding granites, or flanking the numerous protrusions of green- 

 stone within their boundary. They appear like the remnants of overlying 

 rocks among the greenstones, which have escaped the denuding effects of 

 causes that partially stripped tliis region of similar rocks previous to the 

 completion of its present elevation" (p. 546). "The talcose slates are of 

 many varieties, such as would result from their passing into argillitic and 

 into hornblende slates." The greenstone and hornblende-slates occupy only 

 a small portion of the area mapped in PI. I, though they formed the largest 

 portion of the area sm-veyed. The greenstones are described as more or 

 less granular and syenitic rocks, with a dark color when moist. Their com- 

 position is hornblende, feldspar, and quartz, with the hornblende largely 

 predominating, sometimes almost to the exclusion of the other constituents. 

 The hornblende-slates are fine-grained and compact varieties of the green- 

 stone, possessing a "laminated or slaty structure." The slates are cut by 

 quartz veins and by later trap dikes Avhich are supposed to unite with the 

 trap range of Keweenaw Point to the west. 



