164 FORMATIONS OF THE 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 
