BLACK RIVER GROUP. 39 



CHAPTER VIII. 



BLACK RIVER. GROUP. 



§ 76. The Black River Group was defined by Lardner Vanuxem, in the Geo- 

 logical Report for the Third District of New York in 1842, and named from its ex- 

 posures on Black River. The name "Black River limestone" was applied to the 

 cliff extending from Boonville through Lewis into Jefferson County, the cliff being 

 composed of the Birdseye limestone of the Mohawk and the rocks upon which the 

 well-characterized Trenton limestone is placed. We find, the name Birdseye lime- 

 stone applied to rocks in the report of 1838, but not in the sense of the name of a 

 Group of rocks, as the term Black River was used in 1842, and if the name had 

 been so used it would necessarily give way to the geographical name. The Birds- 

 eye limestone was distinguished on the Mohawk by its light dove-color, thick lay- 

 ers, and the presence of crystalline particles representing Phytopsis tubulosa or other 

 organisms, which caused the rock to break readily or possess a kind of brittleness, 

 and when broken to clearly show the crystalline spots. This character is not per- 

 sistent in geographical distribution, and the greatest thickness of the rocks is only 

 about 30 feet. The Black River limestone is distinguished by the abundance of 

 Cephalopoda, and especially by remarkably large Ortlioceras, some of which are 10 

 feet in length and a foot in diameter; beside, it has quite an extensive distribution. 

 The thickness on Black River is about 50 feet. 



§ 77. From New York it extends into Vermont, where about 12 or 14 feet in 

 thickness becomes a black, finely granular mass, susceptible of a high polish, and has 

 received the name of the Black Marble of Isle La Motte. In Vermont it rarely 

 exceeds 20 feet in thickness ; but it outcrops in Penuis Valley, Pennsylvania, with 

 greater thickness than it possesses in New York. It crosses into Canada, and forms 

 a belt upon the margin of the Chazy, but rarely attains any great thickness, though 

 on the St. Lawrence, 90 miles below Quebec, it has a thickness of 130 feet. It has 

 been identified by the, presence of gigantic Ortlioceras on the north-west side of 

 Lake Winnipeg ; and its existence has been noted in the Lake Superior region, on 

 St. Mary's, Escanaba, and Menominee Rivers, and on St. Joseph and Sugar Islands. 

 It has been identified at various places in the Appalachian System, but it thins out 

 westwardly and has a limited area of surface distribution. By some it is regarded 

 as a local and peculiar phase of the lower part of the Trenton, or as constituting 

 merely beds of passage from the Chazy to the Trenton, but there are palseonto- 

 logical reasons for retaining the name as a geological subdivision. It contains 

 many species unknown in the Trenton, though others pass up, as the two Groups 

 are conformable, and both represent the deeper oceanic deposits of limestone. 

 But the strongest reason for holding to the geological separation of so small a thick- 

 ness of limestone from other Groups is that the family Orthoceratidce, which com- 

 menced its existence in the Upper Taconic, increased in genera and species in 

 succeeding ages until it reached its maximum development in this Group. Subse- 

 quently, it diminished in number of species and size of specimens, though it found 

 a home in every Group, until it became extinct in the latest Carboniferous epoch. 

 The Oyrtoceratidce and Endoceratidw were highly developed, and the Oomphoceratidos, 



