106 Tertiary Strata of the Atlantic Coast. 
not appear beneath the sand, is at Bean North Carolina. 
The sands here rest upon secondary rocks. 
. The slight elevation of all our Pliocene strata above the level of 
the sea, confirms an observation of Lyell, that the newer Pliocene 
is found at great elevations only in the countries of volcanoes and 
earthquakes. ‘Thus, remote as is our coast generally from the cen- 
tre of volcanic disturbance, it has to a certain degree been affected 
by it; but an earthquake which might have raised the shores of 
Mexico a thousand feet above the former level, would probably af- 
fect our coast in a far less degree. 
_ On the western shore of Maryland, between the Chesapeake Bay 
and Potomac river, are two classes of deposits besides the equiva- 
lent of the London clay or Eocene, the first of which lying most to 
the westward, contains fewer recent species than the other, and is 
well characterised by the gigantic Perna mavillata. This class of 
deposits appears also on the Eastern shore, but although the Perna 
and some other identical shells occur there, the most abundant spe- 
cies are remarkably dissimilar, the proportion of those existing and 
extinct being much the same. Intermediate to those on St. Mary’s 
river, are strata, in which shells now inhabiting the middle and south- 
ern coasts are abundant, but some few extinct species are common 
to all these localities. So far as my observation has extended, the 
Perna mazillata is always associated with few recent species, and 
may be considered characteristic of the older of these Pliocene 
strata. 
If our tertiary formations do not exactly correspond to those of 
Europe, still there can be no objection to using the terms there em- 
ployed, to designate them, inasmuch as they are equally descriptive 
of, and applicable to, the American formations. The following dia- 
gram will show the relative age and the proposed names of all our 
Tertiary beds. 
- Newer Pliocene. —_—_ Nearly all recent species of the coast. 
Medial Pliocene. About 30 per cent of recent species. 
Older Pliocene. Perna maxillata. Very few recent species. 
Miocene. Probably wanting. 
Riocent 2 No recent shells of our coast and a few 
: secondary species. 
_ The localities of what I have here termed Medial Pliocene, are 
as follows: St. Mary’s river, Maryland; Yorktown, Virginia; James 
river, near Smithfall, Va. ; Suffolk, Va. 
