44 UTICA SLATE GROUP. 



CHAPTER X. 



UXICA. SLATE GROUP. 



87. THIS Group was named the Utica Slate from Utica, New York, and 

 quite fully defined as a geological subdivision in 1842 by both Vanuxem and 

 Emmons in their respective reports. It is in typical localities a dark-colored slate 

 or shaly mass, highly charged with carbon, and agreeing in its composition with 

 the dark layers that separate the limestone strata in the Trenton Group. The surface 

 exposure forms a belt resting upon the Trenton, extending from New Jersey across 

 New York into Vermont, passing under Lake Champlain and entering Canada. 

 The greatest thickness in New York is about 600 feet, and in Vermont about 100 

 feet. It exposes considerable surface in Canada, never exceeding 500 feet in 

 thickness, and extends from Lake Huron, where it thins out, to the eastern shores 

 of the continent, appearing on the Saguenay, in Newfoundland, and the Island of 

 Anticosti. It is very fossiliferous, and everywhere characterized by the presence 

 of Triarthrus becki ; and in the vicinity of Ottawa Triarthus spinosus is abundant, and 

 the Scotch fossil, Siphonotreta scotica, occurs. It is often interstratified with thin 

 bands of limestone. 



88. It is exposed in numerous places in the Appalachian System, and at- 

 tains a thickness in Huntingdon County, Pennsylvania, of more than 1,000 feet. 

 It thins out westerly, and loses its character as a black slate before reaching the 

 Ohio River, where it is composed of blue calcareous shales and marls with inter- 

 stratified thin limestones, apparently forming beds of passage from the Trenton to 

 the Hudson River without any want of conformability. The change in its litho- 

 logical characters would have prevented forever its identification in the banks of the 

 Ohio, had it not been for the tell-tale fossils. The abundance of Triarthrus becki 

 and Leptobolus lepis and associate fossils settled the question of its identity. It is 

 unknown farther west, but exists in the Arctic regions as a more or less calcareous 

 slate. The fossils of the greatest geographical distribution, and by .which it may 

 generally be recognized, are Triarthrus becki, Leptobolus lepis, Asaphus canadensis, 

 Lingula progne, and Graptolithus quadrimucronatus. The rocks are composed in part 

 of mechanical sediment, derived from sources east of the Appalachian System, and 

 not almost wholly of shells and the harder parts of animals, as the Trenton is below 

 and the Hudson River above. It thins westerly, and as the mechanical sediment 

 disappears the marine deposits form continuous passage beds from the Trenton to 

 the Hudson River. The strongest reason for its retention as a geological sub- 

 division is found in the fauna with which it abounds; for at many localities, e. g., 

 Cincinnati, Ohio, and Jefferson County, New York, it can only be separated from 

 the Hudson River Group by an arbitrary line; and at other localities, e.g., Deer- 

 field, New York, and in Kentucky, the Trenton Group is so blended with it that 

 the line of demarcation is wholly obscured. The Galena limestone of Northern 

 Illinois, Eastern Iowa, and South-western Wisconsin occupies substantially the 

 same geological position, though its affinities are more closely allied with the Tren- 

 ton, while the relations of this Group are nearer the Hudson River ; beside, none 

 of the characteristic fossils of this Group are found in the Galena, and none of the 



