152 GEOLOGY OF ILLINOIS. 



that they seem to be an aggregated mass solidified with a caleareo-inag- 

 nesian cement. In the old Niagara seas they must have grown in count- 

 less millions, like oysters in a modern oyster bed. Some of these vast 

 slabs would make attractive specimens for the geologist's yard ; but good 

 cabinet specimens are hard to obtain. Along the Niagara ridges and in 

 the ravines, casts of corals turned to silex may be picked up in great * 

 quantities. Among the most common is the well known chain coral, 

 the Holy sites eaten ularia. Favosites Gothlandica, F. favosa, F. Siayar- 

 ensis, Stromatopora concentrica, S. rugosa, Astrocerium venustum, one 

 or two species of CyatliophyUum, stems of Encrinites, and fragments of 

 Otlliocems of at least two species, are all very abundant. 



From the abundance of these silicified corals, coral reefs must have 

 existed in the old Niagara seas, where countless millions of these little 

 animals lived and built, as the modern coral builders raise up from the 

 modern ocean's floor, reefs, atols and islands. 



In these clear coral growing seas, sea weeds or fucoids abounded, and 

 in the Sterling quarries are woven over some of the layers in a perfect 

 net-work. 



Cincinnati Shales. The rocks of this formation, formerly designated as 

 the Hudson river shales, but now known as the Cincinnati group, show 

 surface exposures over a considerable portion of the county. Along the 

 rapids at Sterling on the banks of the river, and at the base of the bluffs 

 under the Niagara quarries already referred to, the various rocks, shales 

 and clayey and bituminous deposits of this formation may be seen. The 

 rapids in the river are to some extent produced by the wearing away 

 of these deposits. They rise at a considerable angle from beneath the 

 Niagara rocks just below the dam. On the south side of the river the 

 formation can hardly be distinguished, but on the north side, a mile 

 above town, it attains a thickness of thirty-seven feet, from the surface 

 of the water to the base of the Niagara limestone. From thence it runs 

 round east and north of Sterling, three or four miles distant from the 

 city, striking off into the large Cincinnati surface exposures in the 

 neighborhood of DR. PENNINGTON'S residence. In this circular belt 

 there are no surface exposures after leaving Rock river, but the wells 

 dug indicate the existence of these shales and shaley limestones. The 

 inevitable blue clay and creamy colored water, oozing from some small 

 ravines above Sterling, are unfailing indications of this deposit, even 

 where no outcrop is visible. 



That high plateau of level prairie between Elkhorn and Eock creeks, 

 and extending from the railroad track to a mile within the limits of 

 Carroll county, except a small portion of the southwest corner, is under- 

 laid by the rocks of this group. This elevated water shed contains 

 large portions of four or five townships. Round its eastern and northern 



