eastp:rn and southern united states. 
129 
Anna Rivers, tributaries of the Pamunkey, Richmond and Petersburg to the North 
Carolina frontier. For a considerable distance south of Petersburg this line has not 
been accurately traced, but its course shows a southwesterly deflection. 
Eocene. — From the line above indicated as forming the western boundary of the 
Tertiary in general the Eocene extends eastward in a narrow meridional strip, with a 
breadth varying from about 5 miles in the south to 12-15 miles in the north. On the 
west in its northern half it abuts upon the Trias, and in its southern half upon rocks 
of Ar.choean age. The absolute thickness of the formation appears nowhere to have 
been determined, and until more satisfactory data as to the amount of dip and the 
exact boundary outcrops be afforded, no reliable calculation can be made. But it 
would appear that in no locality where the beds are exposed does the visible develop- 
ment much exceed 30-40 feet. On the James River between Coggin’s Point and 
City Point the thickness of the exposed strata is about 20 feet, and in the neighbor- 
hood of Mathias’s Point, on the Potomac, opposite Port Tobacco in Maryland, which 
marks the eastern boundary of the formation, the thickness of the strata, as ascer- 
tained by Rogers, was very nearly the same, or 25 feet. 
There is no reason for doubting that the Virginia Eocene deposits are a direct 
continuation of those of Maryland, as the position of the beds would readily suggest, 
and that consequently they represent an equivalent horizon. This conclusion is 
further borne out by the character of the fossil remains, which approximate closely to 
those of the State referred to, and among which we find such prominent large forms 
as Oxtrea compressirostra* Cardita pJanicosta (including Fc?ierma)-rff« axda of Rogers), 
Turriiella Mo/iuiii, CncuUcea gigantea, the related Cucidlaa onochela (Rogers; 
Latiarca idmen of Conrad), the last but doubtfully distinct from the C. craxsatina, 
Lam., from the Bracheux sands of the Paris basin. Tire large saddle-shaped oyster 
{Ostrea aellccformis) which becomes such a prominent feature in the Eocene fauna 
of the more southern States, and one of whose deposits forms the basement la\ei in 
the famous Claiborne bluff on the Alabama River, appears to have- had a much greater 
development here than in Maryland, where the remains of the species are very scanty. 
Miocene.— The Miocene area extends from the Eocene boundary already defined 
to the sea, occupying what in principal part constitutes the tidal districts of the State. 
\s in the case of the Eocene, there can be no question but that the deposits of this 
age form a direct continuation of the similar deposits of Maryland, and that conse- 
quently we have here approximately the same horizon oi^horizons represent ed . 1 he 
. S,K.cimen in the possession of the Academy of Natural Sciences from near Ale^andna right bank of the 
Potomac. Conrad, in his list of the Eocene fossils 
species to Maryland exclusively. Rogers, in his reports pi singularly enough, credits it with a 
(new .series, vols. v and vi), makes freiiuent reference to a may have been 
liosition ill the Miocene. It is not imlikely tha in resemblance. Nor does it seem unlikely that the 
mistaken for the form iu question, to which it rears . . o . ^ t,, come from the Eocene green- 
O. i is mom nearly 0. dUpariU^^ than 0. conipremToUra, although stated 
sands iRogers, vol. v, p. 340 ; vol. vi, 11. xxvii, fig. 1). 
