

JO DAVIESS COUNTY. =^35 



can be collected. These old Niagara seas swarmed with the coral build- 

 and many of the Niagara beds of rock were little else than coral 



I'iiu-innnli (irnp. The green and blue shales and limestones of 

 tin- Cincinnati group underlie the Niagara limestone wherever the latter 

 is developed iii the county. There are not many natural outcrops of 

 these shales, and they never stand out in ledges or rocky exposures, 

 unless where quarries are opened into the covered rocks. Even where 

 quarries are opened into this formation, and then abandoned for a few 

 years, the rapid disintegration soon covers up the rocks with a gently 

 sloping talus. 



The parts of the county underlaid by this formation can be told at a 

 glance. All around the mounds and mound-like elevations ; all around 

 the outer boundary lines of the Niagara formation, up either side of all 

 the valleys of erosion which have cut through it. the gentle slopes ex- 

 tending from the general level of the country up to the base of the bold 

 Niagara exposures, are underlaid by rocks and shales of the Cincinnati 

 group. These slopes may be represented by a narrow baud two or three 

 hundred yards more or less in width, encompassing all the Niagara 

 fields aud outliers in the county, and running up either side of all the 

 valleys that are cut through it. When this is said, the superficial area 

 underlaid by the Cincinnati group is as well indicated as it could be by 

 many pages of description. One or two localities, however, deserve a 

 passing notice. 



At the northern terminus of Terrapin Ridge, near Elizabeth, the milky 

 looking clays and shales are washed and furrowed out by the rains, ex- 

 posing many fine specimens of the hemispherical-shaped coral Chwtetes 

 petropoUtanus. I have found dozens of good specimens of this coral in 

 the clay-washed road at this locality. 



East of Scales' Mound the track of the Illinois Central Railroad is 

 laid for several miles almost upon the top of the Galena limestone. 

 Several rather heavy cuts in that locality show good exposures of the 

 overlying Cincinnati shales. These beds contain in certain layers a very 

 great abundance of minute fossils, principally a small Xucula. 



The general character of this group in Jo Daviess and Carroll coun- 

 ties is almost identical. The upper layers are thin-bedded argillaceous 

 and silicious shales, of a light buff or creamy color. Where thick- 

 bedded enough to quarry, the stones have a kiln-dried dusty appearance. 

 Lower down, the shales become blue or greenish in color, sometimes 

 separated by thin bands of green, marly clay : still lower, some massive 

 strata of a deep ultra-marine blue color may be found, exceedingly hard. 

 and giving out a clear ringing sound when struck with a steel hammer; 



