x asa Ps oe Ve eae OIE 
270 Cincinnati Society of Natural History. 
The correct rule would seem to be, where a group has been named: 
and the fossils have been so described and illustrated, that it may be 
identified elsewhere than at the typical locality, that the law of priority 
should be rigorously enforced. 
The larger subdivisions or formations are world wide in their dis- 
tribution, and it is probable that many of the lesser subdivisions or 
groups, in distant countries, can be brought into conformity or parallel- 
ized; but, at present, we can not hope for unification of nomenclature 
in the latter respect. : 
Experience has shown the impracticability of making lesser subdi- 
visions, for the purposes of geological nomenclature,than groups, though 
it is eminently proper to speak of the marl beds or sandstone layers, 
in any particular group, or of the Glyptocrinus or Orthis beds, at any 
particular locality. Such names are used to characterize the strata at 
the place described, but not in the higher and more extended sense of 
a geological subdivision. | 
Passing now from these preliminary observations, we will briefly re- 
view the distribution, thickness, and paleontological characters, so far 
as the genera are concerned, of all the groups into which the Silurian 
has been divided in North America. A complete review of all the 
paleontological characters would involve an enumeration of 3,560 spe- 
cific names, and so much minute and dry dissertation that we would 
have a large book opened up before us, instead of an essay embodying ~ 
the general facts and the judgment of the writer for special use on this. 
occasion. 
THE LOWER SILURIAN. 
The Lower Silurian is subdivided, in ascending order, into the Pots- 
dam, Calciferous, Quebec, Chazy, Black River, Trenton, Utica Slate, 
and Hudson River Groups. There are other groups, having a local ex- 
istence, that can not be exactly correlated with these, though, no doubt, 
included within them, which will be mentioned as we progress. 
In 1838, Prof. Ebenezer Emmons described the petrography of the 
sandstone, at Potsdam, St. Lawrence county, New York, which he 
found of considerable thickness, containing fossils, and uniformly 
overlaid by the Calciferous sandrock of Eaton. He traced it over St. 
Lawrence and Essex counties, and proposed for it the designation 
“Potsdam Sandstone.” It was subsequently thoroughiy described in — 
the New York Reports and other State surveys, and shown to embrace 
rocks other than sandstones; but it was not until 1863 that it was : 
