238 Notes on American Geology. 



palaeontology, we cannot say with absolute certainty, in every in- 

 stance, which species originated with any given stratum above 

 the first sedimentary rock ; but generally, shells, corals and plants, 

 which have been continued from one epoch to another, were di- 

 minished greatly in numbers, as if the diminished temperature 

 had been unsuited to their organization. I do not conceive it ne- 

 cessary, as M. Agassiz supposes, to infer that in every grand geo- 

 logical epoch, the fall of temperature was so great as to destroy 

 every species existing at the time, but that some were, like the 

 human frame, more capable of resisting the influence of cold than 

 others. Among living testacea we find some species of a particu- 

 lar genus confined to the tropical seas, whilst others range from 

 the tropics to the 42° of north latitude. The Lucitia divaricata 

 is a remarkable instance of this ability to endure great changes of 

 temperature : originating, as it did, in the Eocene period, it lived 

 in both those of the older and newer Pliocene, and now exists on 

 the coasts of Europe and America, and inhabits the seas of the 

 West Indies, and has been found as far north as Rhode Island. 



We consider those fossils which most abound, when neither 

 broken nor water-worn, to characterize the formation in which 

 they occur, and such as are very rare, to be non-charaCteristic, or 

 accidental, as they may have been introduced with the debris of 

 rocks of an earlier date. Thus we find fragments of Isotelus gi- 

 gas and Calytnene Blumenhachii in the limestone shale at Roch- 

 ester, N. Y., which rock has evidently been derived from the 

 shales of the Trenton limestone formation, and thus fragments 

 of the trilobites of the latter period were swept into the sea, 

 where the shales at Rochester were in process of deposition ; and 

 it is worthy of notice, that the current must have been very gen- 

 tle, judging not only from the fine materials of the shale, but be- 

 cause it has carried only the lightest animal remains, as the thin 

 crusts of trilobites, and rarely any of the small shells which 

 abound in the Trenton shales. Another formation illustrates this 

 fact in a still more satisfactory manner. At Upper Marlborough 

 and Piscataway, in Maryland, a deposit of the Eocene period 

 occurs, composed of the detritus of green sand, a material origina- 

 ting in the cretaceous epoch. One fossil of the latter formation, 

 ( Gryphcea vomer*) is not uncommon among the Eocene fossils. 



* Ostrea lateralis, Wilson. 



