THE ARCII^IAN ROCKS. 497 



blocks by close jointing and containing 28.63 per cent, of Iron, 48 feet. Feet. 

 Returning now to the river bank below, we note next; 



XIV. Magnesian schist: (502, 503) 60 



XV. Covered: by sandstone. Immediately north of the magnesian schist No. 



XIV, and resting cb'rectly against it, horizontal sandstone is seen and con- 

 tinues to show in mural exposures 10 to 40 feet in height, all along the 

 west bank of the river to a point beyond the limits of the section The 

 width of the gap is about 3,500 



XVI. Ferruginous quartz-schist: much oxidized, containing 32.91 per cent, of 



iron 8 



XVII. Mica slate: finely laminated; very light- colored; the mica in bright-lus- 

 tered plates up to % inch in diameter 5 



XVIII. Ferruginous quartz-schist (1017): fine-grained, veiy thinly and dis- 

 tinctly laminated, without contortion; brownish to grayish black in color; 

 non-magnetic; streak red; under the lens seen to consist of mingled 

 grains of white quartz, and a metallic-lustered black mineral (hematite ?) ; 

 contains iron, 32.49 per cent 32 



XIX. Covered 22 



XX. Ferruginous quartz-schist: resembling No. XVIII 7 



Nos. XVI, XVII, XVIII, and XX, all show plainly a strike of N. 65 

 W., and dip of 70 N. E. With the exception of XVIII, which rises 

 10 feet from the river, they are all seen on very low exposures barely ris- 

 ing from the water, and overlaid by heavy beds of sandstone, which shows 

 in a perpendicular face in the bank above. The exact junction of No. 

 XVIII with the overlying sandstone is well exposed, it being possible to 

 obtain hand specimens showing both formations (1009). One of these 

 has already been figured on page 462. Another remarkable feature in 

 these exposures is the bending of the sandstone layers above, to conform 

 with the irregular surface of the schistose rocks, suggesting the idea that 

 a motion upward of the schist had caused the bending, which is seen, not 

 only in the lower layers, but also in the heavy ones 6 to 10 feet above. 

 This feature is represented in Fig. 21. 



At the junction of the two rocks, the sand is seen to have frequently been 

 wedged between the partly separated schist laminae, and in one place in- 

 cludes a detached mass of the schist. 



XXI. Covered by sandstone , 150 



XXII. Ferruginous quartz-schist 3 



XXIII. Covered 40 



XXIV. Ferruginous quartz-schist (519) 5 



Total horizontal width of the slaty series measured 5,406 



Approximate thickness 5, 000 



Nos. XXII and XXI V occur on the east bank of the river, and are barely 

 seen above the water's edge, being overlaid by heavy beds of sandstone. 



The existence, in the region about Black River Falls, of isolated hills of ferruginous 

 schist, which rise through the surrounding horizontal sandstone, has already been alluded 

 to. These hills are known locally as "iron ore mounds." They are from 100 to 250 

 feet in height, and rise somewhat abruptly from the level sandy plain, which is also 

 dotted by loftier castellated outliers composed of higher layers of the same sandstone as 

 that which forms its basement. The rocks of the " iron ore mounds " strictly come un- 

 der another head, being isolated areas of Archaean; but are conveniently alluded to here 

 Wis. SUK. 32. 



