102 KEMP'S ORE DEPOSITS. 



For the Marquette region this has also been further divided by 

 Wadsworth into two, the Holyoke and the Negaunee formations. 

 It is much less metamorphosed than the lower member, and in the 

 Marquette district contains some ore. In the Menominee region 

 of Wisconsin it affords the deposits there wrought and carries the 

 ore in the Gogebic range. Still higher, after another unconformity 

 follows the Keweenawan (Keweenian) or Nipigon. This closes 

 the Algonkian. Still above is the Potsdam sandstone. 



2.02.18. Example 9a. Marquette District. The Marquette 

 district was earliest known and has been most thoroughly studied ; 

 but owing to the confused geological structure, there has been, as 

 already remarked, much discordance of interpretation. In the Mar- 

 quette district the Huronian Algonkian rocks form a broad syn- 

 clinal trough with many subordinate folds and several tongues or 

 projections running out from the main body. They rest on and are 

 bounded by Laurentian gneiss. They consist of green schists, 

 quartzites, banded jaspers, slates, ore bodies, and dikes altered to 

 "soapstone" or " soaprock." Brooks divided them into twenty 

 members, of which Beds VI., X., XIII., and a horizon below Y. 

 afford the ore. Bed XIII. contains the magnetite, which increases 

 in amount toward the western portion of the field. Rominger in 

 Vol. IV. of the Michigan Survey, 1884, reduced the number to 

 seven. Irving and Van Hise have contributed much in late years 

 toward a solution of the geology. Irving regarded the series as 

 separable into a lower division of greenstone schists and more 

 acidic rocks, both of which are dynamically metamorphosed erup- 

 tive rocks, and an unconformable, overlying, iron-bearing division 

 of sedimentary origin. (See papers cited below.) Van Hise, how- 

 ever, in his latest paper places the break above the most important 

 ore bodies. 



2.02.19. The ores were classed by Brooks under five heads (a) 

 Red specular, (b) Magnetic, (c) Mixed, (d) Soft Hematites, and (e) 

 Flag ores ; and the grouping illustrates very well their general 

 characters. Class (a) includes the slaty hematites that break into ir- 

 regular tapering plates, and the massive so-called granular ores. 

 Class (b) includes granular and more or less friable magnetites, 

 which are related to those described under Example 13. Class (c) 

 includes the highly siliceous ores, consisting of hematite, closely 

 interlaminated with red or white jasper. When containing over 

 50$ iron they are at present valuable, but the advance in con- 

 centration, especially magnetic, promises to make the low-grade 



