164 



FORMATIONS OF 



T H E 



INTERIOR 



The best pines for lumber are procured on Snake and Kettle Rivers, and other 

 tributaries of the St. Croix. Those on the main stream are, for the most part, 

 small. 



Sixteen miles below Snake River, rapids commence again, and trap boulders 

 become more numerous, some of them of large size. These rapids mark the place 

 of the last range of intrusive rocks, viz., those which form the rapids, falls, and 

 Dalles of the St. Croix, about thirty miles above its confluence with the Mississippi, 

 represented by the wood-cut on p. 142. It consists of several subordinate ranges, 

 belonging to the same general outburst, which vary from a hundred and fifty to three 

 hundred and sixty feet in elevation : one, which crosses the river at the Falls, two 

 or three above the Falls, and three or four below. Half a mile below the Falls, one 

 of these ranges rises into perpendicular walls on both sides of the river, and consti- 

 tutes the Dalles of that stream. Between these, the St. Croix rushes, at first, with 

 great velocity, forming a succession of whirlpools, until it makes a sudden bend ; 

 then glides along placidly, reflecting in its deep waters the dark image of the 

 columnar masses as they rise towering above each other to the height of a hundred 

 to a hundred and seventy feet. The above-mentioned wood-cut and Section No. 3 

 convey an idea of the appearance and position of this range of trap below the Dalles 

 of the St. Croix. It is one of the finest expositions of that kind of rock which I 

 witnessed in the Chippewa Land District. 



On the west side of the St. Croix, at the Dalles, forty or fifty feet above the pre- 

 sent level of the river, are large pot-holes, some of which are twenty to twenty-five 

 feet in diameter, and fifteen to twenty feet deep. These seem to have been worn 

 into the solid rock by sand, gravel, and loose rocks, kept in motion by circular 

 currents of water, similar to those now observed in the river at the head of the 

 Dalles. They afford evidence, either of successive upheavals of the trap, or of the 

 waters of the St. Croix having flowed formerly at a higher level. 



Immediately at the Falls of St. Croix, the trap rock has nearly a homogeneous 

 character ; but on the high ridges on the west side of the river it is porphyritic, 

 more so than any of the trappean ranges which came under my observation in Wis- 

 consin. On Partridge Ridge, one mile west of the Falls, I observed a variety, the 

 base of which is of a rich dark green, with embedded light pink lenticular crystals 

 of felspar, and disseminated spots of epidote. This porphyritic trap differs but little 

 from the Norway porphyry, found on the west side of the Christiana Fiord, near 

 Bogstadt. 



I caused a specimen of the St. Croix porphyry to be polished. It cuts easily, 

 and its colours show beautifully ; but in consequence of the epidote being softer 

 than the basis of the rock, it receives an unequal polish, which diminishes its value 

 for ornamental purposes. 



Including the intervals between these trap ranges, they occupy a belt of country 

 from fifteen to twenty miles in width. The outburst at the Falls of St. Croix, as 

 heretofore remarked, has forced its way through highly fossiliferous strata, breaking 

 up the beds immediately overlying it, entangling and partially indurating the 

 fragments, without, however, tilting or metamorphosing the adjacent beds in any 

 perceptible degree. The fossils, even of the beds almost in contact with the trap 



