TERTIARY FORMATION. 
269 
the newer and older pliocene, the miocene, and eo- 
cene periods. 
The neiver Pliocene. — Mr. Conrad has pointed out 
a distinct deposite of this formation in Maryland. It 
is well characterized about three miles north of the 
low sandy point which forms the southern extrem- 
ity of the peninsula near the mouth of the Potomac. 
Here the bank rises to an elevation of some fifteen 
feet at its highest point, and the fossils are visible 
to the extent of a quarter of a mile. The stratum 
consists of sand and clay, disposed in horizontal 
beds, with an abundance of shells similar to exist- 
ing species. The distance from the nearest point 
on the Atlantic Ocean is about 45 miles, but nearly 
100 miles by the course of the bay. The extent of 
this formation is not known. 
Older Pliocene and Miocene Formations. — These 
tertiary beds show themselves in a wide and, at 
present, an undefined belt in the southern extrem- 
ity of New-Jersey, and extend through Delaware, 
Maryland, Virginia, and North Carolina, in the 
southern part of which last state, and in part of 
South Carolina, they only occur in interrupted 
patches, thinning out and disappearing altogether 
after reaching the Santee River in South Carolina. 
In New-Jersey, Maryland, and Virginia, the pro- 
portion of recent to extinct species among the fos- 
sils hitherto discovered does not, in the average, ex- 
ceed 20 to 25 per centum, which, therefore, places 
their origin in the miocene era. While the fossils 
south of Virginia show that the southern portions 
of the tertiary deposites belong to the older pliocene 
era. South of Virginia the tertiary beds contain 
nearly two thirds recent species, while north of that 
limit they contain a less proportion of living species 
than one fifth. The total number of species of 
shells found in our miocene formation is about 200, 
40 of which only are living shells, and inhabit the 
adjacent coast. 
Z 2 
