THE ARCHAEAN ROCKS. 477 



tSO7 B 8O7 C 807 A 



Silica 16.33 4- 62.50 = 78.83 



Alumina 12.03 -t- 1.40 = 13.43 



Iron oxide 24 + .50 = .74 



Lime 00 + .64 = .64 



Magnesia 00 -r .07 = .07 



Potash 12 + .25 = .37 



Soda 03 + .04 = .07 



Water 3.75 + 1.70 5.45 



32.50 + 67.10 = 99.60 



The most ferruginous clays seen at Mr. Garrison's yielded 1.68 per cent. (808), and 

 2.31 per cent. (809) of iron sesquioxide. These are apparently much more plenty than 

 the white clay. About 10 rods from the kaolin openings, on the river edge, is a low 

 outcrop of a highly micaceous, weathering gneiss (803). having a moderately coarse, 

 jagged texture. The felspar of this rock is largely still brilliant, but little white kaolin 

 patches dot the surface. Another outcrop near by shows a more highly felspathic kind, 

 with very coarse, pinkish orthoclase. These gneisses closely resemble the prevailing 

 ones in the Grand Rapids section, but are evidently much lower in the series than any 

 of those. 



At the Green Bay and Minnesota depot, Grand Rapids, it is reported that in excavat- 

 ing for a turn-table, first a few layers of compact sandstone were penetrated, then 5 to 

 6 feet of soft white clay and decomposed rock. Near the center of Sec. 4,.T. 22, R. 6 E. 

 (point E of map), about two miles above Grand Rapids, on the east bank of the Wis- 

 consin, on Mr. Rablin's land, very white kaolin shows, overlaid by two feet of sand- 

 stone. This kaolin has been used with success to line the furnaces at the Grand Rapids 

 foundry. The following are analyses of samples from here : 



829 828 1-2 



A. JB B 



Oxide of iron 4.43 



Potash 1.21 .87 .38 



Soda .46 .08 



829 A and 829 B are raw and washed clay taken from the stock-pile at the foundry ; 

 828% is washed from a sample taken from the opening itself. 



On the line of the Wisconsin Valley Railroad, between Centralia and Junction 

 City, are several low cuttings, which expose usually crumbling, and partially decom- 

 posed, laminated gneiesic rocks. The exposures are very poor and the rock is generally 

 out of position. About 3% miles north of Centralia is a cutting 400 feet long, through 

 a rather fine-grained, granular textured, pinkish granite (965). This rock consists of 

 brownish, translucent, granular, glassy quartz, largely predominating; pinkish bright- 

 lustered felspar; and fine black mica sparsely but uniformly scattered. It would dress 

 readily, but shows some tendency to weather and iron stain. 



West of the railroad line, in the western part of T. 23, R. 6 E., sandstone occurs in 

 places, sometimes capping the hills, sometimes low in the valleys, and lying evidently 

 upon a very irregular crystalline rock surface. On Sec. 8, near the northwest corner of 

 the section, a well passes through sand 6 feet, sandstone 2% feet, soft red and white 

 kaolinized rock 20 feet. This is the greatest depth of softened rock that has come to my 

 notice in Wisconsin. 



