COTTONWOOD AND JACKSON COUNTIES. 499 



Potsdam quartzytc.] 



the shores of lakes, along the large creeks, and especially along the whole 

 extent of the Des Moines river. The following species of trees and shrubs 

 are found at Talcott lake: American or white elm, bur oak/ white ash, 

 box-elder, black walnut, willows, prickly ash, smooth sumach, frost grape, 

 Virginia creeper, climbing bitter-sweet, wild plum, choke-cherry, black 

 raspberry, rose, thorn, smooth wild gooseberry, and wolf berry, common; 

 red or slippery elm, cottonwood, hackberry, waahoo, and black currant, 

 less frequent. Basswood grows at Oaks lake, a few miles farther north. 

 About Spirit lake, which lies in the north edge of Iowa and extends into 

 the south part of section 36, Minneota, the timber consists principally of 

 bur oak, white and red elm, white ash, basswood, sugar maple, box-elder, 

 black walnut and cottonwood. 



GEOLOGICAL STRUCTURE. 



Potsdam quartzyte. The only exposures of bed-rock in this district are 

 the red quartzyte which forms a prominent ridge in the north part of Cotton- 

 wood county, reaching into the edge of Watonwan and Brown counties. 

 From the most eastern to the most western outcrop of this rock is a length of 

 twenty-three miles; and the width upon which it is occasionally exposed 

 increases from a half mile or less at the east to six miles at the west. The 

 contour of this area has already been described as rising in a massive highland 

 of rock, mostly covered by a smooth sheet of till, with gracefully rounded top 

 and moderate slopes. The general character of this formation, and the loca- 

 tion, extent, and special features of its outcropping ledges are to be noted 

 here. 



In Courtland, two miles east of New Ulm and about thirty miles east-northeast from this 

 ridge in northern Cottonwood county, and again in Pipestone and Bock counties, fifty miles west- 

 southwest from this ridge, the same rock-formation has extensive exposures, and it continues 

 westward in Dakota to Dell Rapids and Sioux Falls on the Big Sioux river, and to Kockport on 

 the James rive?, seventy miles west of Minnesota, and about a hundred and eighty miles west- 

 southwest from New Him. All these outcrops are mainly very hard, fine-grained quartzyte, dif- 

 fering in color from pinkish gray to dark dull red, always having some red tint; and varying in 

 the thickness of its beds from a few inches, or sometimes only a half inch or less, to one or two 

 feet. It is usually perceptibly tilted, with considerable variability in the direction of its dips, 

 which vary in amount from one or two to fifteen or twenty degrees, and rarely attain an inclina- 

 tion of forty-five degrees. This quartzyte is a metamorphosed sandstone. At a few places it 

 occurs in an imperfectly indurate! condition, being a more or less crumbling sandrock, composed 

 of water-rounded grains. Sometimes, too, it is a conglomerate, enclosing abundant water-worn 

 pebbles up to an inch in diameter, what was originally an ordinary fine gravel having become so 

 cemented as to form a very compact and hard, tough rock; and by diminution in the number of 

 pebbles scattered through it, the formation exhibits all grades between this pudding-stone and its 



