XIII.] GEOGNOSY OF THE APPALACHIANS. 261 



A similar condition of things extends northeastward along 

 the Appalachian region. On the south side of the St. Law- 

 rence below Quebec a great thickness of limestones, sandstones, 

 and slates, formerly referred to the Quebec group, is now re- 

 garded by Billings as, in part at least, of the Potsdam forma- 

 tion ; while on the coast of Labrador and in northern New- 

 foundland the same formation, characterized by the same fossils 

 as in Vermont, is largely developed, attaining in some parts, 

 according to Murray, a thickness of 3,000 feet or more. Along 

 the northern coast of the island it is nearly horizontal, and 

 appears to be conformably overlaid by about 4,000 feet of 

 fossiliferous strata representing the Calciferous sand-rock and 

 the succeeding Levis formation. 



Mr. Billings has described a section from the Laurentian of 

 Crown Point, New York, to Cornwall, Vermont, from which it 

 appears that to the eastward of a dislocation which brings up 

 the Potsdam to overlie the higher members of the Champlain 

 division, the Potsdam is itself overlaid, at a small angle, by a 

 great mass of limestones representing the Calciferous, and hav- 

 ing at the summit some of the characteristic fossils of the 

 Levis formation. Next in ascending order are not less than 

 2,000 feet of limestones with Trenton fossils (embracing prob- 

 ably the Chazy division), while to the east of this the Levis 

 again appears, including the white Stockbridge limestones.* 

 We have here an evidence that the augmentation in volume 

 observed in the lower members of the Champlain division in 

 the Appalachian region extends to the Trenton, which to the 

 west of Lake Champlain is represented, the Chazy included, 

 by not more than 500 feet of limestone. The Potsdam, in the 

 latter region, consists of from 500 to 700 feet of sandstone 

 holding Conocephalites and Lingulella, and overlaid by 300 

 feet of magnesian limestone, the so-called Calciferous sand-rock. 

 In the valley of the Mississippi these two formations in Iowa, 

 Missouri, and Texas are represented by from 800 to 1,300 feet of 

 sandstones and magnesian limestones ; while in the Black Hills 



* T. S. Hunt on the Geology of Vermont, American Journal of Science 

 (2), XLVI. 227. 



