110 The Position of the Cincinnati Group. 



gritty slate, which, even in its unaltered condition, has a cleavage ver- 

 tical to the line of deposition, and is generally destitute of fossils ; but 

 with surfaces covered with curved, fucoid-like marking, which have 

 given it its name. This rock constitutes beds of passage from the 

 Oriskany sandstone, and graduates above into the Schoharie grit, 

 which is an arenaceous limestone, weathering to a brownish color, and 

 succeeded by the gray, subcrystalline, coralline formation, which is 

 known in New York as the Onandaga limestone, while the Cornifer- 

 ous limestone consists of the higher, dark-colored chert beds of the 

 group. (Hall's Pal., vol. iii., p. 43.) 



The Cauda galli grit takes its name from a fucoid, having some re- 

 semblance in form to the tail of a chicken cock. It is 70 feet thick 

 in New York, where the Schoharie grit is only four feet in thickness, 

 but in Pennsylvania and in New Jersey, northeast of the Delaware 

 water gap, these two groups, called the Post Meridian grits, are 300 

 feet in thickness. (Geo. of Penn., vol. i., p. 107.) 



The Onandaga limestone is only from 20 to 50 feet thick in New 

 York, and, though traced over a great extent of country, it rarely ex- 

 ceeds that thickness. In Missouri it varies from 10 inches to 75 feet 

 in thickness. (Geo. of Mo., p. 108.) 



The Corniferous limestone, takes its name from the chert found in 

 it that breaks with a horny fracture. It varies from 100 to 200 feet 

 in thickness in Ohio, Illinois, Pennsylvania, and New York. It is 

 from 300 to 400 feet thick in Michigan, and 850 feet thick in Tilson- 

 burg, Canada. (Geo. of Can., 1866, p. 268.) 



The maximum of these subdivisions of the Upper Helderburg Group 

 is therefore 1,225 feet. Each subdivision in New York is character- 

 ized by distinctive fossils, but in Canada several of the most charac- 

 teristic species belonging to the Oriskany sandstone ascend through 

 each of the overlying subdivisions into the Corniferous Group. 



The Hamilton Group, in its fullest development, consists of the Mar- 

 cellus shale, Ludlowville shale, Encrinal limestone, Moscow shale, Tul- 

 ly limestone, and Genessee slate. 



Prof. Hall says : 



"The Hamilton Group consists, in Eastern New York, at base of the 

 black Marcellus shale, including some bands, of goniatite limestone. 

 Next succeeds a hard, compact, calcareo arenaceous shale, which, under 

 atmospheric influences, crumbles into angular fragments. This is fol- 

 lowed by more arenaceous bands, and by bands of soft, slaty shale, with 

 arenaceous shale or argillaceous sandstone, and with some thin bands of 

 limestone, which are almost entirely composed of organic remains. 

 Toward the western part of New York, the coarser materials gradually 



