THE ARCHAEAN ROCKS. 



499 



nesia. Though obtained on carefully selected, samples, the above figures are probably 

 somewhat high. , Whilst iron ores are worked with even lower percentages than these, 

 Buoh admixtures as quartz and magnesian sflicates would necessitate quantities of iron 

 at least half as large again. The Black river " ores " then, really cannot be regarded as 

 ores, but aie properly iron-bearing rocks. Whetlier valuable working ores may yet be 

 discovered ia these slaty rooks is another question. Similar rocks occur with the work- 

 able ores of Michigan. Taking, however, all the circumstances into account, it is deemed 

 rather improbable that such ores can exist. Even if they do, they are not likely to be 

 discovered, but rather to remain hidden underneath the sandstone that forms the surface 

 rook tliroughoufc tlie region. 



It has been said on a previous page that tlie peculiai Kthological characters of the 

 slaty rocks of Black river, and of the mounds of the vicinity, strongly suggest their 

 Huronian age — a suggestion which is partly corroborated by their position on the border 

 of the great Archaean area of the north part of the state. It has been supposed that 

 "the granite and gneiss of the foregoing section were Laurentian, the slaty rocks Huro- 

 nian. Prom the details given it will be seen that all must be assigned to the same 

 series. 



At Black River Station, on Sec. 3, T. 22, B. 3 W., where the Green Bay and Min- 

 nesota Railway crosses Black river, crystalline rocks are exjiosed in the side and bottom 

 of the gorge through which the river passes, and are overlaid at the top of the banks by 

 a few thin layers of sandstone. The river here trends about S. 25° W., or in a direc- 

 tion roughly at right angles to the general strike. The southernmost exposure examined 

 was about a quarter of a mile below the raiboad bridge. Beginning with it, and pass- 

 inff northward on the west bajik of the river, the following .different rocks were noticed : 



Fig. 22. 



CONTOBTED GnBISS ON BLACK ElVER. 



I Gneiss ■ showing m a rounded knob some 25 feet above the water, and about 100 feet 

 long, and in the river bed below for about 200 feet northward. At the southern 

 end of the exposure the gneiss (1,000) is very fine-grained, thinly laminated, pink- 

 ish-weathered, and quartzose; consisting of flne-granular, glassy quartz, predom- 

 inatmg; fine pinkish felspar, and fine black mica, arranged in lines, the larama- 

 tiou of the rock being also independent of the arrangement of the mica; havmg 

 a strike of N 35° W., and a dip of 62° N. E. A hundred feet northwai-d this 

 merges into a kind (1,001) in which the granular quartz still more largely predom- 

 inates, and the mica is almost wholly absent. A short distance beyond tins changes 

 again to a dark colored, beautifully contorted kind (1,002), consistmg of fine-gramed 



