Trenton limestone. 225 



TRILOBITES OF TUE INFERIOR STRATA. 



It is ill tlic Trenton limestono tliat 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 Ciiazy limestone are confined to a limited 

 district, and lo a small iliickncss of sirata ; and their condition, also, is such as to render 

 their characters indistinct and unsatisfactory. In the Birdsoye limestone, we are forced to 

 depend on a few fragments for the determination of all we know in tliat 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 tlie Black-river limestone. 

 If we were to designate tiiis rock by its most striking, abundant, and peculiar fossils, 

 it would very appropriately be teirncd the Older Trilobile 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 ciiaracteri'/e the rock, at least throughout New- 

 York, Canada, and some of the Western States. Those species in a iowtu' 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 ; Imt it lias already Ijeen sliown how iiUimatcly related are these two formations, 

 constituting, in fact, liut a single natural group. This group, in its western extension, is 

 characterized throughout by the presence of Triloijites, whicii, in New- York, are almost 

 entirely restricted to the Trenton limestone. On this accoiuU, I have thought it lieiter 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 [jeriod 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 sliown when we continue our observations to the western 

 extension of the same formation. These facts should be lionie 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 



