Tlie Position of the Cincinnati Group. Ill 



diminish, and we find an increasing proportion of soft shales, with a more 

 general diffusion of the calcareous raatterj and the mass is terminated 

 by a limestone. Finally, from the Genessee river to the western limits 

 of the State, the entire group, above the Marcellus shale, which is per- 

 sistent, consists of dark, soft shales and bands of limestone. Thus the 

 lithological characters are at the east, an olive shale and "sandstone ; 

 at the west, a grayish-blue, calcareous shale, with bands of limestone. 

 (Pal., vol. iii., p. 46.) 



The Hamilton Group is more than 1,000 feet thick in Eastern New 

 York, and 1,150 feet in Eastern Pennsylvania. (Geo. of Penn., vol. i., 

 p. 107.) It thins out westerly to 50 feet in Missouri, and 100 feet in 

 Tennessee, but maintains a thickness in Canada of from 300 to 600 

 feet. 



The Portage Group is 1,400 feet thick in Eastern New York, and 

 1,700 feet thick at Huntingdon, Pennsylvania. (Geo. of Penn., vol. 

 i., p. 108.) It thins out westerly and northerly. The black slate, or 

 Huron shale, in Northern Ohio, is a part of this group, and has an aver- 

 age thickness of from 300 to 400 feet. 



The Chemung Group is about 2,000 feet thick in Eastern New York, 

 but at Huntingdon, Pennsylvania, it is 3,200 feet in thickness. (Geo. 

 of Penn., vol. i., p. 108.) It thins out westerly to 400 feet in North- 

 ern Ohio, and'200 feet in Missouri. While the Hamilton, Portage, and 

 Chemung Groups in New York are, combined, only about 4,000 feet in 

 thickness, and in Pennsylvania do not much exceed 6,000 feet, at 

 Gaspe, Canada, they are 7,036 feet, though this estimate may include 

 the Catskill Group, 



The passage from the Silurian formation to the Devonian, at Gaspe, 

 Canada, where the rocks are exposed 9,000 feet in thickness, is not 

 evidenced by any change in the lithological character of the rocks, and 

 is hardly determinable from examination of the fossils. The lower 

 2,000 feet is classed with the Helderberg Group in the Silurian, but it 

 may include the Oriskany sandstone of the Devonian series. The up- 

 per 7,036 feet are supposed to represent all the other groups in the 

 Devonian formation of New York, but the divisions are not clearly 

 marked as in New York, nor readily separable from an examination of 

 either the fossils or the rocks. (Geo. of Can., 1863, p. 396 ; do., 1866, 

 p. 260; Hall's Pal., vol. iii., p. 45.) 



The shales and sandstones of the Catskill Group form in their great- 

 est-expansion, at the Catskill Mountains, a mass of at least 3,000 feet in 

 thickness. The group is composed of red and greenish or olive shales 

 and shaly sandstones, with some gray and mottled sandstones and 

 conglomerates. (Pal. of N. Y., vol. iii., p. 51.) 



