500 THE GEOLOGY OP MINNESOTA. 



[Potsdam quartiyte. 



typical condition as a quartzyte. Again, it occasionally contains layers, from less than an inch to 

 several feet thick, of argillaceous rock, so fine-grained and even in its texture as to appear macro- 

 scopically homogeneous, doubtless metamorphosed from deposits of fine silt or clay in the midst 

 of beds of sand; commonly dull red, but often mottled with pale spots or striped by the same lighter 

 tints in parallelism with its stratification; soft enough to be easily carved and polished, and in its 

 best varieties entirely free from grit. This has been named catlinite, and its finest layer is that 

 which has been worked by the Indians, to whom it is still reserved, at the celebrated RedPipestone 

 quarry. 



The planes of bedding of this quartzyte frequently show very distinct and beautiful ripple 

 marks, such as are made by waves upon the sandy shore and bottom of lakes or of the sea. No 

 fossils have been detected in this formation, as here described in southwestern Minnesota and 

 southeastern Dakota: and f ucoid impressions,'rarely observed, are the only remains of life yet found 

 in the probably equivalent Cupriferous series of red quartzytes and sandstones interstratified with 

 thick basaltic overflows and beds of tuff and tufaceous conglomerate, which is very extensively 

 developed about lake Superior. The quartzyte from New Ulm to the James river is closely like 

 the sandstone and quattzyte associated with trap rocks in northeastern Minnesota, in northern 

 Wisconsin and northern Michigan; but its deposition was not similarly accompanied by outflows of 

 igneous rock, nor has this formation in southern Minnesota been intersected by trap dikes. Foster 

 and Whitney referred these rocks in the region of lake Superior to the Potsdam age, considering 

 them the western equivalent and representative of the Potsdam sandstone in New York; and the 

 explorations by this survey of their continuation into northeastern Minnesota sustain this conclu- 

 sion,* while the observations of this quartzyte outcropping in the southwest part of the state and 

 farther west indicate that it belongs to the same epoch. This formation underlies the Calciferous 

 or Lower Magnesian series, which outcrops along the lower part of the Minnesota river from a 

 point fourteen miles east-southeast of New Ulm, and along the Saint Croix and Mississippi rivers. 



In the N. E. J of section 25, Selma, this red quartzyte is exposed upon an eastward slope of 

 till, with an area three rods long from northwest to southeast, and about a rod wide, rising some 

 two feet above the general surface. 



In the S. E. J of section 23, Selma, this rock outcrops on a southward slope along a distance 

 of about twenty-five rods from east to west, with a width of two or three rods and a hight of only 

 one to two feet. It dips about ten degrees southward. Both these ledges have been slightly 

 quarried. They are the ordinary, very hard quartzyte, intersected by systems of joints which give 

 it a rhomboidal fracture. Other outcrops of the same stone, which have not been visited in this 

 survey, occur northwestward at numerous places in this township and in the northeast part of 

 Delton, upon the high ridge and in the hollow where the north branch of the North fork of Wat- 

 onwan river crosses it. 



The quartzyte also has frequent exposures in Delton along nearly the whole extent of the 

 Little Cottonwood river through this township, and in its tributary ravines. In the east part of 

 the S. E. i of section 8, it has been much quarried in the banks and channel of this stream, sup- 

 plying rough stone used for foundations, cellar walls, well curbing and culverts, or, by Russian 

 immigrants, for chimneys, being sometimes teamed fifteen miles. It occurs in layers of all thick- 

 nesses up to two and a half feet, the thinly bedded portions, as usually, being much divided by 

 joints into rhomboidal fragments a foot or less in length. The bedding planes are often ripple- 

 marked over several square rods together, in parallel undulations about a quarter of an inch 

 high and two to four inches apart from crest to crest. The dip is about 5 S. 20 W. This is 

 some twenty rods east of the Little Cottonwood falls, where the same rock in its upper portion 

 forms layers three to six feet thick, dipping about six degrees to the south, but only a few feet 

 lower, uear the level of the stream, is thin-bedded and somewhat contorted and irregular in strat- 

 ification. 



Quartzyte outcropping in the north part of the S. W. J of section 18, Delton occurs in layers 

 up to six inches thick, dipping about 3 S. 70 E. Twenty rods farther south it has a dip of the 

 same amount but changed in direction to S. 40 E., all these bearings being referred to the true 

 meridian. Its only exposures observed in the south half of this township are in the S. E. t of 

 section 30, where it is visible at numerous places along an extent of about an eighth of a mile in 



Consult Prof. Winchell's article on " Tha Potsdam sandstone," in the tenth annual report, pp. 123136. 



