390 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



than in the rocks below ; but its lithology is variable. The same 

 agents were working in the Chemung and the Portage. 



Place. — The Chemung group may be termed one of the " con- 

 stants " of the Central North American palaeozoic basin. Through- 

 out its vast extent, embracing a diameter of 1500 miles, speaking 

 roughly, this stratum is brought under human cognizance wherever 

 there is an uplift of sufficient power to remove the superincumbent 

 rocks ; and this takes place abundantly in the Western States, 

 especially in Illinois and on the River Mississippi, as will be shown 

 incidentally while treating on another subject. 



Together with Marcellus Shale, the Hamilton, and other Devonian 

 groups, the Chemung forms the base and part of the mass of the 

 Catskill Mountains; it likewise occupies the top of a part of the Helder- 

 bergs. It ranges, with its usual associates, from the Delaware River, 

 near Carpenter's Point, along the west side of Mamakating Valley, to 

 Kingston ; thence, on the west side of the Hudson Valley, nearly 

 north, at the distance of a few miles from the Hudson, to the south 

 frontier-line of Albany County, whence it bears away to the north- 

 west, on the Helderberg Mountains, at a higher level than in other 

 parts of their range, and finally, west, along the southern side of 

 the Mohawk Valley ; and it occupies much of the south part of 

 the State between the Erie Canal and Pennsylvania (Mather, Rep. 



The counties in New York in which the Chemung group prevails 

 are Sullivan, Ulster, Chenango, Courtland, Chemung, Steuben, 

 Alleghany, Cattaraugus, and Chatauque. 



In the Western States, it is in narrow bands of enormous length, 

 which follow and underlie Carboniferous Limestone in innumerable 

 windings, and rest upon black slate (Marcellus Slate) or the Hamil- 

 ton group. It is most conspicuous on the Mississippi, as surrounding 

 the Illinois and Michigan coal-fields ; in the Valley of the Cumber- 

 land River, besides many other places. On these points we shall 

 soon have further information from Professors Safford and Swallow. 



Position. — It exhibits undulations, usually slight ; but on the 

 Mississippi River, in two localities of considerable size, the Chemung 

 group is violently subverted by volcanic agency, and in other places 

 it is raised for a short space into flat domes. 



Thickness. — It is 1500 feet thick in New York, thinning, however, 

 westwards. In Pennsylvania, Professor H. D. Rogers makes this 

 rock 3200 feet thick. In that district of country, the Chemung 

 displays some most instructive sections. 



Fossils. — This group contains 104 species of fossils, many or 

 most of them casts (De Verneuil). They often occur in patches or 

 groups. For instance, we have the following fossils in the green or 

 olive shales and shaly sandstones of Rockville and Phillipsburg on 

 the River Genesee, according to De Verneuil (loc. cit. p. 661) : — 



Pterinaea? suborbicularis ; Pecten duplicatus; P. cancellatus; P. ? stri- 

 atus ; P. crenulatus ; P. convexus ; P. dolabriforuiis ; Lima rugostriata ; 

 L. glabra; L. obsoleta; Avicula signata. 



