570 THE GEOLOGY OP MINNESOTA. 



[Gneiss and granite. Kaolin 



From the small creek a mile farther east in section 26, Swede's Forest, ledges of gneiss and 

 granite abound in this valley through a distance of twelve miles, to the mouths of Redwood river 

 and Beaver creek. They often quite fill the bottomland, occuring on each side of the river, and 

 rising 50 to 125 feet above it. Between lledwood river and Beaver creek, frequent small ledges 

 rise along the bottom of the Minnesota valley, in knobs 40 to 60 feet above the river, but yet 

 leave much open, tillable land. Between Beaver and Birch Cooley creeks the outcrops are main- 

 ly on the north side of the river, rising 100 feet in their highest portions. Below the mouth of 

 Birch Cooley they are mostly on the south side, occurring in great abundance for two miles above 

 and three miles below the mouth of Wabashaw creek. The highest of these are a mile above this 

 creek, rising 75 to 125 or perhaps 140 feet above the river. It will be remembered that the bluffs 

 along all this part of the valley are about 175 feet high, so that none of these ledges were visible 

 until the surface of the drift-sheet had been considerably channeled. 



On the Redwood river where it enters the Minnesota valley, at Birum's mill, one and a half 

 miles northeast from Eedwood Falls, the rock is greenish, being apparently a "talcose quartz- 

 yte," or protogine gneiss, dipping 25 S. E. It forms cliffs 50 to 75 feet high, which are continu- 

 ous on the west side of the river a quarter of a mile or more. The picturesque gorge of the Red- 

 wood river, at and below Redwood Falls, is principally cut through a similar gneiss, partly de- 

 composed, and sometimes almost completely kaolinized, overlain by Cretaceous strata, which in 

 turn are capped with glacial drift. The largest cascade, having a fall of about 25 feet, is over a 

 ledge of this protogine gneiss, much contorted and jointed, often obscure in its lamination. The 

 dip of the principal system of joints, which appears to coincide nearly with the lamination, is 20 

 to 30 N. At a cut which has-been made through the rock two rods east of this cascade, it con- 

 tains a nearly vertical trap dike, seen along an extent of some thirty or forty feet, bearing N. 40 

 E., about two feet wide, composed of dark and greenish, compact rock, which weathers to a red- 

 dish color, much jointed in planes parallel with its walls. Ten feet above the bottom of this cut, 

 and higher, the cliff of gneiss is much decayed and changed to impure kaolin. 



In Brown county no exposures of the Eozoic rocks have been examined, but their character 

 has been learned from their outcrops along the northeast side of the Minnesota river, in Ren- 

 ville and Nicollet counties, under which they are fully described. Their outcrops in Brown 

 county are of small extent, including only a few localities on the bottomland of the Minnesota 

 valley along the northern boundary of Eden and Home townships. Their extent southeastward 

 is to "Little rock," about five miles below Fort Ridgely, beyond which the only outcrop of these 

 rocks in the Minnesota valley is a small area of granite opposite the southeast part of New Ulm. 



Decomposed gneiss and granite. Very remarkable chemical changes have 

 taken place in the upper portions of many of the exposures of gneiss and 

 granite near Redwood Falls. The rock is transformed to a soft, earthy or 

 clayey mass, resembling kaolin. It has a blue or greenish color, when 

 freshly exposed; but when weathered, assumes a yellowish ash color, and 

 finally becomes white and glistening. Laminae of quartz are generally con- 

 tained in this material, and have the same arrangement as in gneiss, so that 

 the dip can be distinctly seen. Veins of quartz or feldspar, the latter com- 

 pletely decomposed, and the lines of joints, are also noticeable, just as in 

 granite or gneiss; making it evident that this substance is the result of a 

 decay of the rocks in their original place. Because of the enclosed quartz- 

 ose laminas, grains, and particles, of more or less gritty character, through- 

 out these kaolin-like rocks, they appear to be unsuited for the manufacture 

 of porcelain or any kind of ware. So far as can be judged from stream 



