. Report of a geological survey of Wisconsin, Iowa, and Minnesota : and incidentally of a portion of Nebraska Territory : made under instructions from the United States Treasury Department . eing filled, generally, with reaching the dike (No. 362), a quartz vein occurs, with native copperdisseminated through the veinstone (No. 367). Three hundred yards above the last-named dike, the river is crossed by another,bearing northeast and southwest, over which there is a fall of eight or ten these last two dikes, the strata of metamorphosed sandstone and shales(No. 363)
. Report of a geological survey of Wisconsin, Iowa, and Minnesota : and incidentally of a portion of Nebraska Territory : made under instructions from the United States Treasury Department . eing filled, generally, with reaching the dike (No. 362), a quartz vein occurs, with native copperdisseminated through the veinstone (No. 367). Three hundred yards above the last-named dike, the river is crossed by another,bearing northeast and southwest, over which there is a fall of eight or ten these last two dikes, the strata of metamorphosed sandstone and shales(No. 363) are bent upwards, and form an arch about twenty-five feet in lower beds, which disintegrate easily, have been worn out by the action of theriver, and a cavern of some depth has been formed under the arched rocks, asshown in the sketch below. On one side of the arch the dip is to the southeast;on the other, to the northwest. Beyond this point, for the distance of a mile, the river runs in the line of bearingalong the side of the dike, which is the only rock exposed, as far as explorationwas made. Among the debris of this river, I saw numerous fragments of veinstone contain-. ARCH OF METAMORPHOSED SANDSTONE AND SHALE. » NORTHWEST LAKE S U P E 111 0 E. 353 ing native copper, but was only able to discover the vein spoken of above. I canonly say, in reference to the metalliferous indications in the rocks of this river, thatI consider them sufficiently important to deserve the attention of the miner. The point at the mouth of Knife River is made by No. 609. The bay belowthe mouth of the river is lined by an amygdaloid (No. 010), so full of cells that itfractures with the greatest ease. It extends along the whole of the bay, and bearsnorth 30° east. It is more amygdaloidal at some points than others; and, as thefirst point is approached, the cells contain a great deal of thalite. At the point, therock is red, like No. 599, and is bedded, though much disturbed. Just around
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