THE CINCINNATI SHALES AND LIMESTONE. 315 



of the Galena limestone, and the upper limits of the Lower Silurian 

 formation. 



General Character. The shales that form so prominent a member 

 of this group are of a varied character. One kind is little else than a 

 slightly indurated green or blue clay, often very fine in texture, con- 

 taining but little sand, or other hard material, and being, chemically, 

 little else than silicate of alumina. This graduates, by the addition of 

 fossils, iron pyrites, calcareous and silicious matter, and, occasionally, 

 crystals of gypsum, into a variety of impure and changeable shales. 

 Another class has :i more slaty structure, having been deposited in 

 delicate laminse of fine sediment, not so largely clay as the above. 

 These split with great ease arid regularity into thin, brittle plates. 

 Still another kind lias a more arenaceous character and regular bed- 

 ding, giving it somewhat the appearance of a fine grained sandstone. 

 These three general classes include many minor varieties. 



The associated limestone is also varying in nature. A portion has 

 a coarse, granular, crystalline texture, mingled with earthy, or ocher- 

 ous matter, while another portion is compact, crystalline, with leaves 

 of argillaceous matter variously distributed through it; and still other 

 portions are silicious or cherty. These several forms often give place 

 to, or graduate into shale. The limestone occurs at all observed hori- 

 zons, but, except at the northern extremity of the exposure, is much 

 less in quantity than the shale. Near Little Sturgeon Bay, the cal- 

 careous material is much increased, and limestone predominates. 



The upper part of the formation is chiefly composed of the clay- 

 shales and limestone. 



The slaty and arenaceous shales are found at lower horizons. Their 

 special positions and distribution may be found among the local de- 

 tails. 



The thickness of the formation may be placed at 200 feet, with an 

 ascertained variation from 165 feet to 240 feet. There are reasons for 

 believing that the extreme range is somewhat greater. 



Life. Sea weeds represented the plants, and Radiates, Mollusks, 

 and Articulates, the animals. 



The most noteworthy feature is the remarkable abundance of Chsete- 

 toid Corals and Bryozoans at certain localities. Upwards of 30 species 

 were collected from the shale thrown out of two shafts of no consid- 

 erable depth. Other Corals, than those related to Chaetetes, are far less 

 abundant. Brachiopods are next in prevalence, the genera Orthis and 

 Strophomena predominating. Lamellibranchs and Gasteropods are 

 rare, and Crustaceans and Crinoids limited to a few species. More 

 specific facts will be given with the local details, and a full list, ex- 



