70 GEOLOGY OF ILLINOIS. 



geodiferous shales, may be seen in the bed of McGee's creek, and they have 

 afforded a few of the characteristic fossils of this formation, among which, 

 were Agaricocrinus Americanus, Archimedes Owcnana, and Spirifer Keokuk. 

 This limestone outcrops along the bed of the creek, at intervals, as far west as 

 township 1 south, range 5 west, in Adams county, the easterly dip of the strata 

 corresponding very nearly to the fall of the creek. 



At Jaqueth's mill, about six miles a little south of 1 west from Versailles, the 

 geodiferous shales of this group are well exposed, forming the base of the 

 bluff, as shown in the following measured section, made at this point : 



FEET. 



Fine grained sandstone 6 



Brown shales 10 to 15 



Brown magnesian limestone 8 " 10 



Blue shales, with geodes 35 " 40 



The lower bed in the above section consists of blue argillaceous shales, trav- 

 ersed by perpendicular veins of satin spar, from a quarter of an inch to an 

 inch in thickness. The geodes from this locality contain beautiful crystals 

 of brown and colorless calcite, dog-tooth spar, zinc blende, dolomite, iron py- 

 rites, and the more common forms of crystallized quartz and chalcedony. They 

 are mostly of small size in the bluff at the mill, but at other points they are 

 larger and are mostly lined with quartz crystals. The regularly bedded gray 

 limestones, which form the lower portion of this group, do not appear above the 

 surface in this county, but would be found a few feet below the level of the 

 main water courses. 



Economical Geology. 



Coal. As has already been stated, the Coal Measures underlie nearly all 

 the uplands in this county, and attain a maximum thickness of nearly one 

 hundred and fifty feet, including the horizon of the three lower coal seams. 

 Only one of these, however, No. 2, or the Colchester coal, of McDonough county, 

 appears to be generally developed in this county, and from this, nearly all the 

 coal mined at the present time is obtained. We found this seam very uniform 

 in its thickness, and apparently extending over nearly the whole area underlaid 

 by the Coal Measures. It affords a coal of good quality, and the only draw- 

 back to the success of coal mining enterprises in this county, is the thickness 

 of the strata, which varies from twenty-four to thirty inches. The roof is 

 generally a clay shale, though, at some localities, the lower part of it becomes 

 highly bituminous, passing into a black shale, which forms an excellent roof. 

 This seam is only worked in a very primitive way, by the process called " strip, 

 ping," which consists of throwing off the overlying material, where the coal 

 outcrops in the valleys of the small streams, and then taking out the coal 



