498 



GEOLOGY OF CENTRAL WISCONSIN. 



on account of their close relations to, and short distance from, the slaty rocks just de- 

 scribed as occurring on Black river. Tor the positions of the various mounds, see Atlaa 

 Plate XV, Area F. The exposures observed on them are mostly poor, and nearly always 

 of ferruginous quartz-schist, hke that occurring on Black river, and quartz. On those 

 mounds, however, which lie on sections 15 and 14, T. 21," R. 3 W., and Sec. 31, T. 22, 

 R. 3 W., the iron oxide, instead of being magnetic, or red, or brown and hydrated, is 

 biilKant specular hematite. The specimens from the mound on Sec. 12, T. 21, R. 4 W., 

 show chiefly a dark-colored magnetic rook, like that of Tllden's iron mound. On the 

 mound on Sec. 17, T. 21, R. 3 W., white quartz only was observed. 



The considerable amount of iron in the schists of Black river, and of the mounds in 

 the neighboring country, has for many years attracted attention to those rocks, it being 

 supposed that they were of value as ores of iron. Several attempts at smelting have been 

 made. One smaU furnace was built on the banks of Levin's creek, as long ago as 1855, 

 and another begun, but never completed, on the south side of Tilden's mound, near the 

 river. In the first-named, the ferruginous quartz-schist of the north side of Tilden's 

 mound was mixed with the hematitic magnesian schists from the west side of the same 

 mound on the river bank. As a flux for this mixture, a dolomitic limestone from the Lower 

 Magnesian formation was used. It may be readily seen that no successful work was 

 ever done. 



In view of the considerable interest that had been excited with regard to these ores, 

 and the reputation they had aheady attained, the writer was sent, during the first year 



Pig. 21. 



Potsdam Sandstone on AncHiBAN Schists. 



XVI PeiTUginous quartz-scliist, 8 feet. XVII Mica Schist, b feet. XVIII Ferruginous scliist, 

 23 feet. XX Formginous schist, 7 feet. A, mass of .Schist inclnded in tl\e sandstone. BB, Pots- 

 dam sandstone, tpi) layer 3 .feet thiclc. Scale, 20 feet to 1 inch. C. D., River level. 



of the present survey, by the then Chief Geologist, Dr. I. A. Lapham, to make an exam- 

 ination as to their value! Sairiples'for analysis were averaged from all the exposures 

 and in all the openings seen; and analyses made of most of the samples, especially with 

 regard to their richness in iron. . The ores or .iron-bearing rocks are of two general 

 kinds: the ferruginous quartz-schists, in which the iron-bearing ingredient is at difi'erent 

 times magnetite, specular hematite, red hematite, and the brown or hydrated oxide — ■ 

 the last two probably from weathering only — and the ferruginous magnesian schists, 

 in which the iron oxide is red hematite. Of the former kind, the various samples yielded 

 respectively, 26.04, 26.98, 28.63, 29.17, 30.23, 30.90, 31.87, 32.10, .32.49, 32.91, 34.22, 

 35.96, 37.18 and 42.52 per cents, of iuetalhc iron. Of the latter, observed only in one 

 place, the samples yielded 9.81, 28.13 and 31.27 per cents.' In the first kind, the only 

 other important ingredient besides iron oxide is quartz; in the second, a silicate of mag- 



1 For the " hard " or silicious ores of Michigan, 50 per cent, of iron is the minimum amount at 

 which the ores can find purchasers. 



