16 GEOLOGY OF ILLINOIS. 



referring doubtless to the prevailing fossils. The Petitame- 

 rux ob/oitynx is met with in great abundance in its upper 

 beds, and its corals are so numerous, and finely preserved 

 in the form of silicified casts, as to show us that the Xiagara 

 seas were the coral-paved seas of the Silurian age. 



The Cincinnati Group. Xext in the descending order 

 comes that group of clayey, unctious, fine-grained shales, 

 formerly called the Hudson river shales by most of our 

 Western geologists, but now more appropriately named 

 by our own State geologist. The upper parts of the quar- 

 , ries show thin-bedded stratifications, but towards the bot- 

 toms of the quarries the strata become thick-bedded and 

 solid. The thin shales are light-yellow, buif or green col- 

 ored, soft, sometimes unctious to the feel, often giving a 

 creamy color to the water, as it trickles down from the 

 quarries, and crumbling and melting into clays when ex- 

 posed to atmospheric influences. Some of the massive 

 strata near the bottom of the formation are intensely hard 

 and very blue. 



The maximum thickness of this formation reaches per- 

 haps a hundred feet. Above Savanna the outcrop is eighty 

 feet in thickness, and at Blutfville a like thickness is ex- 

 posed ; and a well, thirty feet deep, near by, exposes to the 

 bottom the shales and clays of this formation. In a few 

 places the thin, cream-colored strata break into rhomboidal, 

 diamond-shaped blocks of great regularity. 



In a few localities the shales are almost black, and have 

 so much carbon in their composition as to burn with a 

 bright flame, giving out considerable heat, and resembling 

 cannel coal. The flame resembles that of burning petroleum. 



The fossils are mostly Brachiopoda, and exist in great 

 abundance in some of the strata. 



The stone, even the best of the hard blue, except in cer- 

 tain localities, is utterlv unreliable as a building stone. It 



* - w O 



disintegrates and crumbles on exposure to atmospheric in- 

 fluences. On this account natural exposures are rare. In 



