ed 
On the Tertiary Strata of the Atlantic Coast. 281 
An upheave sufficient to elevate the bar above the surface of the 
ocean would convert what is now sea into a lagoon, and the lagoon 
into dry land. The latter would become a deposit of fossil oyster 
shells, and the newly formed lagoon would gradually be peopled with 
the same oyster should the species be preserved. These tertiary 
deposits are, even now, the substrata of the ocean bed along the coast, 
but the mud and sand have accumulated to such a depth, that the 
fossils are seldom washed up by the surf. It does, however, occa- 
sionally happen that they are cast ashore, much to the surprise of 
the inexperienced conchologist. [I have found specimens of the Ran- 
gia cyrenoides, exceedingly water worn on the coast of Virginia, 
and this shell does not inhabit the open sea, and is at present con- 
fined to the estuaries of the Gulf of Mexico. Professor Ravenel, 
of Charleston, an excellent conchologist, has a specimen of Venus 
alveata, (nobis) an extinct species, which was found on the sea beach 
of Sullivan’s island, washed up doubtless by the waves. 
As the Ostrea virginiana originated in the Miocene era, it may 
be that the fragments of Pecten Madisonius at Easton, were recent 
exuvie, and that every tertiary deposit above the Eocene contained 
in the shallow and tranquil waters, beds of living oysters. It may 
also have happened that sand bars increesed in elevation during the 
lapse of ages, until in places, the intermediate waters were protected 
from the sea, and thus strata of oceanic shells would be formed, cap- 
ped by the beds of oysters; both, in that case, were probably rais- 
ed to their present elevation by a single upheave. 
In the newer Pliocene of the Potomac river we find a deposit of 
such shells as now live both in the open sea, and in the. harbors of 
Newport, New York, and Charleston. The Pholas costata is there 
imbedded entire in the clay, im precisely the same manner as the 
living shell burrows,* but when the oysters made their appearance, 
these oceanic shells no longer inhabited this particular locality, and 
it is obvious that some cause had rendered it unfit for their peculiar 
habits; with the appearance of the oysters, the clay was no longer 
deposited, but sand and gravel began to be washed into the shallow- 
ing estuary. The oysters however, seem to have lived here for a 
short period only, for they constitute but a thin seam, and their shal- 
* The clay of the Newer Pliocene period is atte similar to that now occur- 
ring inthe sea. I have seen a specimen of the latter from the New Jersey coast 
full of young sdie{is of Pholas crispata, and the same ‘tind of clay is washed ashore 
from the Gulf of Mexico with specimens of Pholades imbedded in it 
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Vol. XXVIII.—No. 2 
