14 STEATIGEAPHICAL GEOLOGY. 



which have commenced an existence in the lowest forms of marine fish, soon to 

 appear in higher states of perfection, and to be followed by a number of Ba- 

 trachian or Reptilian forms before the close of palaaozoic time. 



The Hamilton Group was named by Vanuxem, from Hamilton, Madison 

 county. New York. In its fullest ilcvolopment, it consists of the Marcellus 

 shale, Ludlowville shale, Encriiial limestone, Moscow shale, Tully limestone, and 

 Genessee slate. It is about 1,200 feet thick in Eastern New York, and 1,150 feet 

 in Eastern Pennsylvania. It thins out Westerly and Southerly, but maintains 

 a thickness of from 300 to 600 feet in Canada. 



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 atmos- 

 pheric influences, crumbles into angular fragnicnts. This is followed by more 

 arenaceous bands, and liy bands of soft slaty shale, with arenaceous shale or ar- 

 gillaceous sandstone, and with some thin bands of limestone, which are almost en- 

 tirely composed of organic remains. Toward the Western part of New York the 

 coarser materials gradually diminish, and we find an increasing proportion of 

 soft shales, with a more general diffusion of the calcareous matter, and thrfmass 

 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 persis- 

 tent, 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 Portage Group was nametl from Portage, New York. It is 1,400 feet 

 thick in the Eastern part of the State, and 1,150 feet thick in Eastern Pennsylva- 

 nia. It thins out Westerly and Northerly. The Black Slate or Huron Shale of 

 Michigan and Ohio belongs to this Group, and is from 300 to 400 feet thick. It 

 is from 100 to 200 feet thick at Louisville, Kentucky, and New Albany, Indiana, 

 and 50 feet thick in Missouri. The Group ought not to be called the Huron Shale 

 either in Ohio or elsewhere, because that name was appropriated by the early 

 Canadian geologists, and applied to a Group of Metamorphic rocks. If one 

 were to speak of the slate in the Huronian Group, it would be called the Huron 

 Slate, and so would shale if found there, be properly designated as Huron Shale. 

 The words Huron and Huronian have the same signification, and are too near alike 

 to be used to designate widely separated Groups of rocks. The " Huron Shale " 

 is a synonym for the " Portage Group," with nothing to commend its use. because 

 it has neither geographical nor local significance. 



The Chemung Group was namcil from Chemung, New York, and is about 

 2,000 feet thick in the Eastern part of that State, but at Huntington, Pennsylvania, 

 it is 3,200 feet in thickness. It thins out to the West, and is estimated at only 

 400 feet in Ohio, where it is called the Erie Shale, and 200 feet in Missouri. 

 While the Hamilton, Portage and Chemung Groups in New York are, combined, 

 only about 4,000 feet thick, and in Pennsylvania do not much exceed 6,000, 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 



