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, 

 such admixtures as quartz and magnesian silicates would necessitate quantities of iron 

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

 ores, but are properly iron- bearing rocks. Whether valuable working ores may yet be 

 discovered in these slaty rocks 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 

 rock throughout the region. 



It has been said on a previous page that the peculiar lithological 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. From 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, R. 3 W., where the Green Bay and Min- 

 nesota Railway crosses Black river, crystalline rocks are exposed 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 railroad bridge. Beginning with it, and pass- 

 ing northward on the west bank of the river, the following different rocks were noticed : 



FIG. 22. 



CONTORTED GNEISS ON BLACK EIVER. 



I. Gneiss: showing in 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 fine-granular, glassy quartz, predom- 

 inating; fine pinkish felspar, and fine black mica, arranged in lines, the lamina- 

 tion of the rock being also independent of the arrangement of the mica; having 

 a strike of N. 35 W., and a dip of 62 N. E. A hundred feet northward 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, this changes 

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



