238 Notes on American Geology. 
' paleontology, 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 soliticmalt from one epoch to ‘nother, were di- 
minished greatly in numbers, as if the diminished temperature 
been unsuited to their organization. Ido 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 Lucina 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 Calymene Blumenbachii in the limestone shale at Roch- 
ester, N. Y., which rock has evidently been derived from the 
: 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 geD- ’ 
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 rienteii 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, 
( Gryphaa vomer,*) is not uncommon among the Eocene fossils. 
- * Ostrea ey Wilson. te 
