148 FORMATIONS OF THE INTERIOR 
form falls or rapids, to partial outcrops in the ridges, between the waters flowing 
into the Mississippi and Lake Superior, and to somewhat more elevated and exten- 
sive ranges along the extreme northern limits of the United States. It is altogether 
probable, however, that at least one-half of the drift region is based on gneiss, mica- 
slate, other metamorphic schists, and granitic rocks; while the other half is under- 
laid by Protozoic sandstones. 
SECTION V. 
THEIR PHYSICAL AND AGRICULTURAL CHARACTER. 
THE principal exposures of Protozoic Strata pertaining to F. 1 and F. 2, which 
have been the subject of the first chapter, extend, as I have there shown, as far as 
the falls of the eastern tributaries of the Mississippi. The average distance of their 
northeast line of limit from that river, in a direct course, may be from fifty to 
seventy miles, or, by the meanders of the streams, ninety to a hundred miles. There 
the crystalline rocks emerge from beneath the water-courses, as seen in the cut on 
page 
This change in the geological formation of the country is accompanied by a cor- 
responding change in its physical features. Just before reaching the igneous ranges, 
the streams are usually hemmed in on either side by solid walls of sandstone (F. 
1, a), known, as already stated, by the name of Dalles. A few miles beyond these 
commences a succession of low falls and rapids, which interrupt navigation, and 
render portages necessary before the traveller can advance even with a bark canoe. 
These obstructions are formed by outbursts of igneous rocks. 
The elevating forces which have brought these to the surface, through the interior 
of the District, have not been sufficient to produce hills of any great height, such as 
are usually to be found in regions of crystalline rocks, that constitute extensive 
water-sheds. On the contrary, though we here approach the dividing ridge, whence 
rise, on one side, many of the most important tributaries of the Mississippi, and on - 
the other, numerous streams emptying into Lake Superior, we find a level country, 
or at least merely a succession of low, flat plains, rising twenty-five to thirty feet, 
one above another,* with intervening ridges one to two hundred feet high. It is 
at the commencement and termination of the former that the principal falls and 
rapids set in. 
The soil prevalent over the drift region under consideration, is, as a general rule, 
especially in the southeast, poor, thin, and seldom above second-rate character. In 
a country of this description, one is, at first, surprised to encounter a multitude of 
lakes, ponds, and swamps, to such an extent as to render travelling, even on horse- 
back, hazardous. This seems little in accordance with the porous, siliceous character 
of the superficial deposits over this region. The explanation is, that these are 
underlaid by clays and argillaceous beds, highly retentive of moisture, and derived, 
* On the Upper Wisconsin, the crystalline rocks are elevated several hundred feet above the river, but 
this is an exception to the general rule through this part of Wisconsin. 
