TRENTON LIMESTONE. 
225 
TRILOBITES OF THE INFERIOR STRATA. 
It is in the Trenton limestone that we are first made acquainted with this class of animals, 
in any considerable number. In the preceding rocks, their remains are exceedingly rare 
and obscure. The few species known in the Chazy limestone are confined to a limited 
district, and to a small thickness of strata ; and their condition, also, is such as to render 
their characters indistinct and unsatisfactory. In the Birdseye limestone, we are forced to 
depend on a few fragments for the determination of all we know in that rock. In the 
Trenton limestone, we have at least fifteen or sixteen well characterized species, and an 
immense number of individuals in a more or less perfect condition. They appear to have 
been as abundant, even, as the Orthocerata of this rock, or of the Black-river limestone. 
If we were to designate this rock by its most striking, abundant, and peculiar fossils, 
it would very appropriately be termed the Older Trilobite limestone ; for nowhere in the 
series do we find, in a single rock or formation, so many species as in this. We may indeed 
confidently rely upon these fossils alone to characterize the rock, at least throughout New- 
York, Canada, and some of the Western States. Those species in a lower position can 
scarcely lead to any confusion, since they are few in number, and rare in all the localities 
examined. 
It is true that several species of this limestone reappear in the shales of the Hudson-river 
group ; but it has already been shown how intimately related are these two formations, 
constituting, in fact, but a 'single natural group. This group, in its western extension, is 
characterized throughout by the presence of Trilobites, which, in New-York, are almost 
entirely restricted to the Trenton limestone. On this account, I have thought it better to 
arrange the species of this rock in connection with those occurring in the succeeding shales. 
This arrangement seems the more necessary, since, in the western extension of this 
group, where the calcareous matter is augmented, many of the species continue throughout 
its entire thickness, terminating only with the deposit itself. The period of the existence of 
certain species in this position is limited in New-York solely by the cessation of calcareous 
deposits ; since it is clearly shown, that in other situations, where the formation continued 
to be calcareous, they existed for a longer time. 
I have already shown, in regard to the Brachiopoda and Monomyaria ( and the same 
is true of other fossils), that the former are far more abundant in the Trenton limestone, in 
the calcareous part of the formation ; while the latter increase in the Hudson-river, or in 
the shaly portion of the group. That all these changes are dependent on the nature of 
the sediment, can be clearly shown when we continue our observations to the western 
extension of the same formation. These facts should be borne in mind by the geological 
student, in his investigations of the New-York strata, and the same succession of strata 
elsewhere. 
The general proposition regarding not only the Trilobites, but other fossils, may be 
[Palaeontology.] 29 
