AZOIC SYSTEM. 169 



upon the oldest, even when there are within its limits more 

 than two thousand feet in thickness of strata, which properly 

 intervene between them in the geological scale. 



Although the dip of the strata at this locality is to the 

 northward, nothing more is seen of the formation to the 

 southward of that point, from which it is inferred that it also 

 dips immediately to the southward again and is lost beneath 

 the Cretaceous and other formations in that direction. 



Going northward, up the river valley from this point, the 

 strata entirely disappear by their northerly dip within half a 

 mile of the southern limit of the exposure; but within five or 

 six miles still further to the northward, they rise again and 

 are seen at intervals along the valley, between there and 

 Sioux Falls, in Dakota territory. The falls are about ten 

 miles in a direct line northwestward from the northwest 

 corner of the State of Iowa. The Big Sioux river, which, by 

 an abrupt bend, has here a northward course, passes over a 

 bold outcrop of the quartzite, causing a series of falls, sixty 

 feet in aggregate height, within the distance of half a mile. 

 The trend of this outcrop is also eastward and westward, 

 but the dip is to the southward, at an angle of six or eight 

 degrees, being in an opposite direction from that of the 

 current of the stream, and in opposite direction also from 

 that of the dip of the same strata at the northwest corner of 

 Iowa. By estimating from the angle of the dip and actual 

 measurement of the height of the falls, we find the formation 

 to have a thickness here of three hundred feet, and yet we 

 saw neither the base nor the well-defined top of it. 



In Rock county, Minnesota, which joins the northern 

 boundary of Iowa and the eastern boundary of Dakota, are 

 other large exposures of the Sioux quartzite. In Pipe-stone 

 county also, which lies immediately north of Rock county, 

 are still other large exposures. The most important of these 

 exposures of quartzite, encloses the famous pipe-stone layer, 

 from which the Indians have manufactured their pipes from 

 time immemorial. This pipe-stone is a bed of metamorphic 



clay, known among mineralogists as Catlinite, about a foot in 



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