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342 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Feb. 21,
Fig. 2.
Section from Gay Head to Sancati.
Island of Martha's Vineyard.
Nantucket.
a. Tertiary clay.
b. Stratified drift (sand and gravel).
c. Oyster-bank.
d. Stratified drift, sand overlying the oyster-bank.
cliiied to consider the tertiary itself as deposited by the same tidal
agency, being formed from the detritus of the greensand of New
Jersey ; a supposition which seems the more probable as it would
account for the great similarity of materials between the greensand of
New Jersey and the tertiary of Long Island, a similarity which is so
striking, that it led even Mr. Mather to consider this tertiary as iden-
tical with the greensand.
Concerning the drift overlying the tertiary clay at Sancati, it is
obvious from the regularity of the strata, and from the very perfect
state of preservation of the shells imbedded in it, that it has not un-
dergone any violent disturbance since their deposition. The species
collected by us in the above-mentioned oyster-bank are the following :
Venus mercenaria, plenty.
Mya arenaria, plenty.
Buccinum undatum, rather rare.
■ plicosum, abundant.
Ostrea borealis, a bed several feet thick. Nassa obsoleta, abundant.
trivittata, abundant.
Scalaria groenlandica, rare.
Balanus rugosus, very abundant.
Serpula, forming a layer several feet thick.
Pagurus pallicaris (claws).
Area transversa, very abundant.
Solen ensis, abundant, but very brittle.
As t arte Cartanea, rather rare.
Cardita borealis, rare.
Cumingia tellinoides, rather rare.
Crepidtda fornicata, abundant.
Now these are, without any exception, the same species that are
found living on the shore of Nantucket and Cape Cod, and as they
are all in their natural position, the bivalves having almost always
the two valves united, and the Venus being commonly half open, just
as they are found on the beaches when the muscles have relaxed after
death, we may fairly infer that in this part of the continent at least,
the climate has not undergone any considerable change since the
deposition of these fossils.
The presence of a stratum of disintegrated shells of the same
species, resting upon the undisturbed oyster-bank, may easily be
accounted for by a somewhat more violent action of the tides, which
deposited in this irregular manner a part of the shells which were
washed off, from the oyster-bank itself, in the same way as is the
case now among the Nantucket shoals. Indeed there is to be found
on the slopes of every shoal ridge, a region from which the dredge
brings up nothing but loose and broken shells. This region is so
characteristic and so constant, that one of us has designated it in his

• ' Report on the Distribution of Animals among the Shoals*," under a
  

peculiar name, as the Recjion of Broken Shells. According to Capt.

• See Proceedings of the Boston Natural History Society.