BROWN AND REDWOOD COUNTIES. 569 



Gneiss and granite.] 



Minnesota valley, about sixteen miles below the last; and on the Cotton- 

 wood river in western Brown county. Fossiliferous and sometimes lignitic 

 clays of Cretaceous age are occasionally encountered in the wells through- 

 out this region, especially at Walnut Grove and northward in western Red- 

 wood county, and in Lyon county, adjoining this on the west. The sheet 

 of drift which forms the surface is thus often separated by unconsolidated 

 Cretaceous beds from the underlying floor of crystalline rocks. Within the 

 area here reported this gneissic and granitic floor outcrops, away from the 

 valley of the Minnesota river, at only one or two points, which are in T. 

 ill, R. 38, Redwood county. These formations will be described in the 

 order of their age, beginning with the oldest. 



Gneiss and granite. These rocks have the same composition, being 

 made up of quartz, feldspar and mica. Gneiss differs from granite in hav- 

 ing these minerals laminated, or arranged more or "less distinctly in layers. 

 Nearly all the metamorphic rocks to be described here are varieties of 

 gneiss, with which masses of granite, syenite and mica and hornblende 

 schists occur rarely. 



In the N. E. J of section 12, T. Ill, R 38, an exposure of rock extends ten rods in length 

 from northwest to southeast, with half as great a width, rising 5 to 10 feet above the surface of 

 the undulating prairie. It is light gray gneiss, much contorted, with its strike and dip obscure; 

 intersected by few joints, which in some portions are absent across an extent of three or four 

 rods; enclosing at the southeast two or three masses of nearly black mica schist, each two or 

 three feet long. 



About five miles farther west, the N. E. \ of the S. E. \ of section 6, in the same township, 

 is said to have an exposure of similar rock, about three rods in extent, with a larger space around 

 it where the rock lies only a few feet beneath the surface. 



The depth of .these rocks in this region is generally from 100 to 200 feet or more, so that 

 they are not reached by wells nor by the channels of most of the rivers. Their only other out- 

 crops in Redwood and Brown counties are within the Minnesota valley, and in the gorge of the 

 Redwood river at and below Redwood Falls. 



The Minnesota valley in the northwest corner of Swede's Forest, and in the edge of Tel- 

 low Medicine county, contains abundant ledges for two miles, reaching 40 to 75 feet above the 

 river. A lone school house is situated among them, about a mile east of the county line. Half a 

 mile west from this school house, the rock is reddish gray gneiss, dipping 15 N. K. W. A third 

 of a mile west from the school house are massive granite cliffs, probably rising 75 feet above the 

 river, divided by joints into nearly square blocks ten to fifteen feet in dimension. This rock may 

 be found valuable for quarrying. An eighth of a mile east from the last, it is obscurely laminated 

 gneiss, much intersected by joints, the principal system of which dips 15 S. At the east side of 

 the school house, it is also gneiss, somewhat water-worn, dipping about 5 S. 



Within the next few miles in following down the river, similar ledges are seen on its north- 

 east side, in the N. E. } of section 16, in Sacred Heart, Renville county, rising about 50 feet above 

 the river; in the southeast part of section 17, Swede's Forest, rising at several points 25 to 40 feet; at 

 south side of Big Spring creek, in section 20 and the west edge of section21, Swede's Forest, about 

 50 feet above the river; and near the north line of section 27, small in area, and only about 20 

 feet high. 



