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 silieious sand and'' 

 a bluish clay, soft and tenacious, arranged in hori- 

 zontal layers. This is often called blue marl, and 

 contains 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. Our descrip- 

 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 hitherto 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 

 of 210 eocene species from Alabama, not more than 



