498 



GEOLOGY OF CENTRAL WISCONSIN. 



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

 scribed as occuning on Black river. For the positions of the various mounds, see Atlas 

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

 of ferruginous quartz-schist, like 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 

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

 show chiefly a dark-colored magnetic rock, like that of Tilden'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 these rocks, it being 

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

 made. One small 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 

 monnd 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 already attained, the writer was sent, during the first year 



Fig. 21. 



B 



ID 



POTSDAM SANDSTONE ON ARCHAEAN SCHISTS. 



XVI Ferruginous quartz-schist, 8 feet. XVII M;ca Schist, 6 feet. XVIII Ferruginous schist, 

 23 feet. XX Ferruginous schist, 7 feet. A, mass of Schist included in the sandstone. BB, Pots- 

 dam sandstone, top layers feet thick. Scale, 31) 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. Samples 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 

 lands: the ferruginous quartz-schists, in which the iron-bearing ingredient is at different 

 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 metallic iron. Of the latter, observed only in one 

 place, the samples yielded 9.81, 28.13 and 31.27 per cents. 1 In the first kind, the only 

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



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

 which the ores can find purchasers. 



