70 PROTOZOIC ROCKS 
Upper Mississippi, is situated in this sandstone. The effect of this cave is striking. 
The colourless aspect of the stone, contrasted with the deep shadows of the interior, 
the dark waters of a copious spring which breaks through the snow-white floor, 
together with the coolness of the atmosphere, call up the idea of an undermined 
glacier or icy arch, giving exit to some mountain torrent. 
This sandstone varies in thickness from forty to one hundred feet. It appears 
to be destitute of organic remains; at least none have as yet come to light. In 
the absence of these it is difficult to say whether it ought to be considered as the 
terminating member of Formation 2, or the inferior member of Formation 3. 
Since, however, it appears to have been produced by a repetition of sedimentary 
action, similar to that which occurred just at the commencement of Formation 2, I 
have thought it best to place it, for the present, as the terminating mass of that 
formation. 
It forms, as already remarked, part of the slope between the first and second 
terrace at Prairie du Chien. Beyond this locality, however, it soon runs out and 
is not again in place, in going north, until within a few miles of the St. Croix. 
Near that river, the Kinnikinick, Willow River, and Apple River, where more 
indurated than usual, it forms outliers, which appear in the shape of curious, 
symmetrical, low, flat hills, which look like artificial mounds. This wood-cut, from 
OUTLIER OF SANDSTONE, KINNIKINICK. 
a sketch by Mr. Lewis, represents an exposure of the upper beds composing this 
sandstone formation, as they appear on a hill near the Kinnikinick, examined by 
Dr. Shumard. He describes it as an isolated mound, surmounted by about forty 
feet of bare ledges of the sandstone in question, capped by a few inches of shell 
limestone on the summit. It is even more difficult to account for the preservation 
of this mass than of those formerly alluded to on the Wisconsin River, because this 
sandstone is, generally, of looser and more incoherent materials than the sandstone 
beneath Formation 2. This is the most easterly outlier of Formation 2 c, and Forma- 
tion 3, on the St. Croix. Several others, with a thicker capping of limestone, are found 
