CHAPTER I. 



FORMATIONS OF THE UPPER MISSISSIPPI AND ITS TRIBUTARIES, BELONGING 



TO THE SILURIAN PERIOD. 



The 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 blufis 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 

 in 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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