270 
TERTIARY FORMATION. 
The miocene beds usually consist of sands and 
clays of various colours and characters, having 
much oxide of iron often mingled with the earthy 
matter, giving it a yellow or brown appearance, es- 
pecially the upper strata. The lower strata are 
generally composed of a green silicious sand and 
a bluish clay, soft and tenacious, arranged in hori- 
zontal layers. This is often called blue marl, and 
cohtains an abundance of brittle, shelly matter, 
which proves a valuable manure in sandy soils. 
Throughout the whole of these beds there is dis- 
seminated an abundance of greenish-black grains 
of the green sand, already described as belonging 
to the cretaceous formation of New-Jersey. 
Eocene Formations. — This oldest division of the 
tertiary extends along the western limit of the At- 
lantic Plain, in a belt of from 10 to 20 miles broad, 
between the primary and secondary rocks and the 
miocene strata, from beneath which the formation 
in question rises westward with a very gentle in- 
clination. Going south, we first strike it in Mary- 
land, between the Chesapeake Bay and the Potomac 
River, as at Fort Washington. Its eastern limit 
crosses the Potomac near Matthias's Point, and pur- 
sues a course almost due south, crossing the James 
River at Coggin's Point, and thence stretching on 
south through all the Southern states to Alabama, 
where it forms an extensive deposite. Ourdescrip- 
tion of the miocene beds will also apply to this for- 
mation, as it is also composed of a loose mixture 
of various coloured sands and clays, abounding in 
ferruginous matter and green sand. The total num- 
ber of eocene shells hitherlo discovered is about 
230, not one of which is common to the miocene 
formation, nor now exists, or can be referred to any 
recent species. In Europe, however, out of 1238 
:species belonging to the eocene, 42 are common to 
the miocene. Another interesting fact is, that out 
eocene species from Alabama, not more than 
