272 Cincinnati Society of Natural History. 
specially characterize any of them, though Orthoceras reached its most 
remarkable development in the Black River Group, and Orthis in the 
Hudson River; Discina and Lingula are genera, probably errone- 
ously determined, but, if correctly, they have lived in all ages since, 
and are, therefore, not characteristic of any group. Theca is found 
in the Potsdam, and continues to occur as high as the Hamilton Group, 
or Middle Devonian. Agnostus and Conocephalites occur in the 
Potsdam, Calciferous, and Quebec Groups. Obolella, Paradoxides, 
and Microdiscus, are Potsdam genera, and possibly indicate that the | 
strata belong to the Lower Potsdam, While the authors have not re- 
ferred any of the forms to the species which have been described from _ 
the Upper Potsdam, nevertheless, there is nothing in the general 
facies, to distinguish it as a distinct group. Indeed, it would be an 
extraordinary and notable occurrence to find the same species in the 
shales and slates at St. John, New Brunswick, and in sandstone at 
places as distant as Tennessee, New York, and Minnesota, even if they 
were of precisely the same age. 
The Potsdam Group, in New York, is usually a hard silicious sand- 
stone, white, red, gray or yellowish, and having a thickness of from 
100 to 200 feet. The lower portion is frequently a granitic conglom- 
erate, in which masses of rounded and water-worn quartz eight or ten 
inches in diameter are enveloped. The thickness of this portion is not 
more than ten feet, but in the extension into Canada it is fully three 
hundred feet. The sandstone differs in texture and aspect at different 
exposures. In some places a dark, slaty sandstone, about ten feet in 
thickness, intervenes between it and the Calciferous; at others, a very 
coarse brecciated rock intervenes; and, at other places, the passage is | 
very gradual into the Calciferous sandrock. It extends from New 
York into Vermont, where its thickness is only from twenty to fifty 
feet, unless other rocks are included in the group than merely the un-— 
altered sandstone. 
It passes from New York into Canada, where it soon attains a thick- — 
ness ranging from 300 to 700 feet, and, at the summit, the sandstone 
becomes by degrees interstratified with beds of Magnesian limestone, 
that constitute a passage to the Calciferous. It rests unconformably — 
upon and fills up the inequalities of the underlying Laurentian forma- 
tion over a great part of the area of its distribution. Below the Chau-— 
diere, it consists, at the base, of red, green, black, and lead-gray shales, — 
and hard arenaceous-calcareous argillites, interstratified with eray 
sandstones, and of gray limestone and limestone conglomerates, inter-— 
