36 GEOLOGY OF ILLINOIS. 



below these there is found in some localities a black carbonaceous 

 shale, so highly charged with carbon as to burn with a bright flame as 

 though impregnated with oil, and the bottom of the deposit is made up of 

 thinner strata of alternating yellow, blue, and green shales and clays. 

 AVherever the rain cuts through the soil into these shales, or the little 

 streams wash them, the wet clays have a greasy look, and the trickling 

 waters a creamy and greenish color. There are no gradual beds of 

 passage into the overlying Niagara or the underlying Galena limestones; 

 but the formation preserves well its distinctive characteristics. The 

 beginnings of its foundation stones and its cap rocks are always easily 

 recognized. 



The thickness of the deposit cannot be accurately stated. A true 

 section, as developed in the Mississippi river bluffs, from Bluffville, in 

 Carroll county, to the mouth of Fever river, would run from eighty to 

 one hundred and twenty feet. In the interior of this county it nowhere 

 perhaps reaches to one hundred feet, and in some places it is only from 

 forty to sixty feet. 



The deposit is full of well preserved fossils. The Orthoceratite beds in 

 Dubuque county, Iowa, have long been famous for the number of well 

 preserved Orthoceraiites with which they are crowded. 



The C licetetes petropoUtanus is a characteristic fossil, and is found in 

 great abundance at Elizabeth, and in the washes and ravines at other 

 places. Fragments of a branching coral, and the small bud-like heads 

 of an encrinite, are generally found in the same localities. In a few 

 places I observed immense numbers of the fragments of Isotelu* yujax ; 

 also several species of Orthis, among them Orthis lynx; associated with 

 Ambonycliia radiata, Stropliomena alternata, fragments of two or three 

 species of Orthocera, and one or two of. the new fossils described in the 

 Third Volume of the Illinois Geological Eeports, Stropliomena unicostata 

 and Tentaculites sterlingensis, were also observed. 



T lie Galena Limestone. This is the great bed-rock of the county. 

 From Dunleith to about the mouth of Small-pox creek it forms the 

 rocky bluffs on the Mississippi river. All the north-western, northern, 

 and north-eastern part of the county, except a few of the mounds here- 

 tofore named, is underlaid by it. The eastern part of the county, ex- 

 tending a short distance south of Morseville, is also underlaid by the 

 same rock. All the larger streams in the county, including Sinsinnewa, 

 Fever, and Apple rivers, Eush, Small-pox and Plum creeks, with their 

 principal tributaries, flow along the surface or cut into this formation. 

 It immediately underlies the surface deposits of something like two- 

 thirds of the county. 



The maximum thickness of the Galena rocks in this county is not 

 known. It is probably not far from three hundred and fifty feet. At 



