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 wc 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, I am 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 amst 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 Whales, where the strata are much 

 disturbed. 



Tlie Pentamerus ohlongus, so well known and extensive in its geograpliical range, is 

 never found in the United States associated with the fossils of tire lower division. On the 

 other liand, 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 Clicmung group ; since, as already shown, 

 there is no lithological change more obvious or important at tlie liase 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 a-s 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 fislies 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 ? 



