XVI INTRODUCTION, 
included in the Wenlock formation of England. Passing upward, on the other hand, to 
the red sandstone, containing remains of peculiar fishes so analogous to those of the 
British Isles, that we unhesitatingly refer the rock to the Old Red Sandstone of Europe ; 
we find the formation separated by an unequivocal line of demarkation from the rocks 
below, which are charged with shells and trilobites. So abrupt and well defined is this line, 
that in undisturbed regions we have no difficulty in recognizing it by the sudden and entire 
cessation of brachiopods alone, while usually the lithological change is more distinctly 
marked by a coarse sandstone or conglomerate. Whatever may be said, therefore, of the 
identity in age, and the mingling in the same formation, of Devonian fossils, such as 
Brachiopoda, Acephala and Gasteropoda, with the peculiar fishes of the Old Red Sandstone 
in Great Britain, such a condition never happens in the United States, so far as observations 
have extended.* 
At the present time, Iam obliged to recognize the following great subdivisions as in- 
dicated by zoological characters. Commencing with the lowest rock known to contain 
fossils, we find the first important change in the typical forms to-occur at the termination 
of the Hudson-river group; which is marked by a coarse sandstone or conglomerate 
(the Oneida conglomerate or Shawangunk grit), beyond which scarcely a single species 
has prolonged its existence. This point must be considered as representing that horizon, 
which, in Great Britain, is the termination of the Lower Silurian deposits. We never find, 
however, in the succeeding groups, a mingling of the fossils of the lower and higher rocks, 
which is regarded as taking place in England and Wales, where the strata are much 
disturbed. ; 
The Pentamerus oblongus, so well known and extensive in its geographical range, is 
never found in the United States associated with the fossils of the lower division. On the 
other hand, it occurs in a calcareous band among shales and sandstones, far more naturally 
belonging to the succeeding higher strata than to the lower. Moreover, although found in 
*In the State of New-York, and in other parts of the United States, the most natural and obvious arrangement 
would be to include in one system all the strata to the termination of the Chemung group; since, as already shown, 
there is no lithological change more obvious or important at the base of these higher formations, than there is at the 
base of the Niagara and Clinton groups. Where the Oriskany sandstone is absent, it is even scarcely possible to 
distinguish the line of separation between the Niagara and Corniferous limestones, though one is regarded as of 
Devonian and the other of Silurian age. If we consider any one class of fossils as a guide in determining the limits of 
systems, then perhaps the peculiar ‘‘ Devonian fishes,” which first appear in our Schoharie grit, or at the base of 
the Onondaga limestone, will be regarded as indication of the commencement of a new era. Still, however, the 
characters of the other classes of fossils is not materially changed, and several species of the central part of the system 
have prolonged their existence into the superior strata. The zoological question, therefore, is to be tested upon the 
ground, whether the commencement of a certain order of fishes at a certain period is more important than the 
subsequent change, when all the other classes of organic remains are exterminated, and the same order of fishes is 
continued ? 
