THE ARCH^AN ROCKS. 



469 



grained, greenish, decomposing, pyritiferous, mioa-sohist (881). At the fall at the up- 

 per end of the channel the rook is hai-d, very iine-grained, light-colored, distinctly banded 

 gneiss (882), containing much quaj.-tz and greenish, greasy mca along the lammation 

 planes. The northeast strike and southeast dip (50°) show here very jjlainly. 



It would appear that we have in these exposures the synclinal hue at which the north- 

 west dip of all the beds further down the liver gives place to the southeast one which 

 prevails for many miles above. 



Continmng now along the west bank of the Wisconsin, we note next an occurrence 

 of kaohn on the land of Mr. L. P. Powers, lot 5, sec. 24, T. 22, R. 5 E. The kaolia 

 IS exposed naturally in the river bank at several pomts, and has been at one place laid 

 bare by digging,' several carloads having been sent away for trial. The exposures are 

 some ten feet above the river at low water, and show an apparent thickness of about 

 three feet. Much of this is pure white, plastic clay, easily removed wath a spade, but 

 in many places, and especially towards the base of the exposure, it grades into a par- 

 tially altered rock of varying degrees of firmness. In much even of the pei-fectly soft 

 clay the lamination planes of the unaltered rook, are still distinctly to he seen, and thin 

 plates of solid quartz remain in theh^ original positions, dipping oast of south, at about 

 65°. Where these lines are so distinct the clay is frequently of a bluish cast, and then 

 appears to be less refractory than usual. AH of it tends to be stained superficially by a 

 brownish oxide of non, which may be due to the infiltration of ferruginous waters, and 

 is not in sufficient quantity to affect the value of the clay. In a few places, however, 

 large dark- reddish patches are seen imbedded in the white clay, and are undoubtedly 

 due to the presence of a large and pernicious quantity of iron oxide. Immediately 

 above the kaohn is found a layer, 13^ feet thick, of quartz fragments, mostly angular, 

 and evidently derived from the decomposing rook just below. Above this in a few 

 places are to be seen one or two thin layers of crumbly, brownish sandstone, a remnant 

 of the sandstone which overlies the crystalline rooks everywhere in the vicinity. Above, 

 again, is found the soil, three feet in thickness. Below the kaolin, at the water's edge, 

 low exposures of unaltered gneiss appear. A series of specimens for analysis obtained 

 from this place yielded the following results : — ■ 



8S3 



834 



SiHca 70.82 



Alumina 18.98 



Iron peroxide 1-24 



Lime 24 



Magnesia 02 



Potash 2.49 



Soda 10 



Water 5.45 



Carbonic acid 02 



1.21 



trace 

 8.84 



2.34 



2.30 

 trace 



1.96 

 trace 

 6.30 



Specific gravity . 



99.36 

 2.55 



.1 



70.25 



17.68 



2.32 



.33 



1.49 



1.69 



.39 



5.61 



99.76 

 2.50 



2.33 

 .10 



8.84 



S23 



69.34 



19.19 



1.75 



.44 



.89 



3.30 



2.4S 



2.67 



99.51 



2.85 



822 A is from the exposure furthest down stream, and was analyzed just as taken 

 from the ground, after drying at 100°. It represents a thickness of about three feet. 

 It will be noticed that the alkali is chiefly potash, a fact which seems to be attributable 

 to the greater difficulty with which the potash felspar decomposes, both potash and soda 

 felspars entering into the composition of the gneisses of the vicinity. The small trace 



' This kaolin has been mncli farther developefl eince the time of my examination in 1874. 



