48 GEOLOGICAL SURVEY OF WISCONSIN. 



2d. " The horizontal sandstones of the Apostle Islands, and the wo?t 

 end of Lake Superior, were laid down subsequent to this tilting, and 

 also to an immense amount of erosion; and the sandstones of eastern 

 Lake Superior were formed at the same time. These points are 

 proved by (1) the occurrence of horizontal sandstones in immediate 

 proximity to tilted sandstones and traps, in Ashland county ; (2) the 

 occurrence of the same In the Apostle Islands, within but a few miles 

 of the tilted beds of the Montreal; (3) the actual contact of the hori- 

 zontal sandstones with the melaphyrs of the Oopper-bearing series in 

 Douglas county; and (4) similar and additional facts observed by 

 Messrs. Brooks and Pumpelly in the upper peninsula of Michigan. 



3d. " That hence the Copper-bearing rocks should rather be classed 

 with the Archaean than with the Silurian rocks." 



Under the head of " Economic Geology," Prof. Irving discusses the 

 questions regarding the probable existence of copper, silver, etc., with 

 an account of the attempts heretofore made in mining for these metalo. 



Accompanying this supplementary report, Prof. Irving has furnished 

 for preservation, a transcript of notes made by him in 1873, on the 

 iron ores and iron mounds at and near Black River Falls, in Jackson 

 county, with the analyses of the ores, so far as they have been finished ; 

 these are to be used in compiling a final report, when the survey in 

 that neighborhood is completed. From these notes it appears that 

 there are no less than nine diflerent beds of iron ore interlaminated 

 with slates, crossing the river at various points, within a distance of 

 less than two miles, and that there are seven prominent mounds, con- 

 sisting chiefly, or largely of iron ores, with slates and quartz. Seve- 

 ral analyses already completed, indicate 26 to 32 per cent, of metallic 

 iron, associated chiefly with silica. The slates are supposed to be of 

 Huronian age, resting upon gneissoid granite below, and covered ua- 

 conformably by the Potsdam sandstone. 



There are indications of a great lapse of time, and of very conside- 

 rable erosion after these slates were hardened into rock and elevated 

 to their present position, but before the deposition of the Potsdam 

 sands. Specimens were obtained near the mouth of Snow creek, show- 

 ing the junction of the sandstone with the Huronian slaty ore, in a 

 very interesting manner; some of the horizontal and some of the in- 

 clined layers being seen in the same hand specimen. 



These notes are accompanied by a map showing the occurrence of 

 the mounds, and another showing the position of the iron beds along 

 the river above the falls, with diagrams illustrating the same. 



Prof. Irving's party took the field in the latter part of April, and 

 continued until the portion of the appropriation allotted to this ser- 



