CALHOUN COUNTY. O 



We are unable to fix the exact period when this disturbance took place, but 

 it seems to have been anterior to the coal epoch. This is indicated by the 

 unconformability of the coal strata to the underlying limestones on the north 

 side of this axis in Pike and Adams counties, where the Coal Measures rest 

 unconformably on the Keokuk and Burlington limestones, showing that these 

 beds had been elevated, and a considerable thickness of strata removed by 

 denudation, before the deposit of the coal. Southwest of this axis, the coal rests 

 on the St. Louis limestone, but whether exactly conformable to it or not, we 

 can not say, from the partial exposures we were able to examine. 



The following section exhibits the different formations that may be seen in 

 this county, showing their relative position and thickness. This section pre- 

 sents a thickness nearly equal to one-half of all the stratified rocks found in 

 the State, and, with the exception of the middle and upper Coal Measures, and 

 the Chester group of the Lower Carboniferous limestone series, it comprises all 

 the important divisions of the paleozoic strata to be found in the State. 



FEKT. 



Loess 40 to 60 



Drift, clay and gravel _ 10 " 20 



Coal Measures *. 100 " 120 



St. Louis limestone 200 



Keokuk group 150 



Burlington limestone 200 



Kinderhook group 120 



Hamilton limestone '. 6 " 15 



Niagara " 50 " 75 



Cincinnati group 80 " 120 



Trenton limestone 350 " 400 



St. Peters Sandstone . . 150 



Maximum thickness .'. . . 1,630 



The St. Peters Sandstone. This is the oldest rock appearing above the sur- 

 face in this county, and its only point of outcrop is at the Cap au Ores bluff, 

 on section 30, town 12 south, range 2 west. It forms the lower escarpment of 

 this bluff, which is about a mile in extent on the river, but it dips strongly to 

 the northeastward and disappears below the succeeding formations so rapidly, 

 that it is nowhere seen except at this point. The lowest portion of the bed 

 does not appear above the surface, but there is about one hundred and fifty feet 

 in thickness exposed at the lower end of the bluff, which gradually passes be- 

 neath the magnesian beds of the Trenton group towards the upper extremity, 

 making its entire outcrop along the river a little more than a mile in length. 

 It is a purely silicious rock, made up of minute grains of quartz that are some- 

 times scarcely cemented at all, and some portions of it readily crumble to sand on 

 exposure. Other portions of the mass are tolerably well cemented by the infil- 

 tration of the oxyd of iron, and the rock then forms a bold mural precipice along 



