CHAPTER I. 
FORMATIONS OF THE UPPER MISSISSIPPI AND ITS TRIBUTARIES, BELONGING 
TO THE SILURIAN PERIOD. 
Tue Report made by me, in the autumn of 1839, of the Geological Survey of the 
Mineral Point District of Wisconsin, the Dubuque District of Iowa, and a portion 
of Northern Illinois, contains a full description of the country bordering on the 
Mississippi, and lying between latitude 41° 30’, and latitude 43°; or in other words, 
as far north as Wisconsin and Turkey Rivers. The terminating sections of that 
Report along the bluffs of these two streams, are taken as a starting-point, whence 
to commence the present description of the geology of the Upper Mississippi, north 
of latitude 43°. Thus the Report in question, taken in connexion with the present 
Report, will comprise an account of the geological features of the State of Iowa, the 
western part of Wisconsin, and a large portion of Minnesota Territory. 
The illustration prefixed to this chapter represents a natural section, and Sect. 
No. 1, A, an artificial section, of the hills at Prairie du Chien; furnishing the order 
of superposition of the stratification, as well as the key of connexion by which to 
unite the survey of 1839 with those of 1847, 48, 49, and ’50. 
The lower terrace or projecting ledge is the upper portion of the Lower Magnesian 
Limestone, which forms the base of these hills, and which extends down to the 
level of the plain on which the village stands. It is the same rock which has been 
used in the construction of the church, and of several other buildings in that place. 
Its thickness, from the quarry at the base of the hill to the top of this projection, 
is about one hundred and sixty feet. The principal part of the slope between this 
and the second terrace, is occupied by soft sandstone, between forty and fifty feet 
im thickness. The second terrace marks the junction of that sandstone with the 
buff, blue, and gray fossiliferous limestone, which is upwards of a hundred feet 
thick, and fills the greater part of the upper slope, capped on the summit by the 
Coscinopora beds of the Magnesian Limestone, or Lead-bearing Rock of the Mineral 
Point, and Dubuque Districts of Wisconsin and Iowa. 
The whole of these strata rest, as stated in my Report of 1839, on the soft, white 
quartzose sandstone near the level of the bed of the Mississippi. 
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