418 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



reduced to one level by the deposition of an incumbent mass, con- 

 sisting chiefly of red clay. The great width of estuaries such as 

 that of the James and other rivers which have narrow channels in 

 the higher and mountainous region, may probably be attributed 

 to the facility with which, on the eastern coast of America, the 

 action of the tides has removed the soft and incoherent tertiary 

 deposit, when the land was gradually emerging from the sea. 

 There is reason to believe, from what we know of the coast of 

 S. Carolina and Georgia, that this emergence was accompanied by 

 oscillations of level, which would greatly facilitate and prolong the 

 period of the denuding operations. 



North Carolina. 



In the cliffs at Wilmington, North Carolina, resting on a cal- 

 careous eocene rock, are seen miocene shelly strata of the ordinary 

 character, in which I collected about thirty species of shells. The 

 recent species bore a larger proportion than usual to the extinct ; 

 but this may be owing to the circumstance that two-thirds of the 

 whole consisted of marine bivalves, which is rather a larger propor- 

 tion than that observed in the fossil miocene fauna, amounting to 

 about 150 species, which I collected in Virginia. I have in- 

 variably found that the proportion of marine bivalves identical 

 with the recent, whether in the English crag, the faluns of 

 Touraine, or other tertiary formations, is greater than that of the 

 marine univalves, a result in perfect harmony with the law of 

 geographical distribution of living mollusca pointed out by 

 M. Philippi, who has shown that the range of species in the marine 

 bivalves is far more extensive than in the univalves. In Touraine 

 I found the per-centage of recent species vary in the faluns as 

 much as 10 per cent., and even more, in different places not very 

 remote from each other ; but when the number in each locality 

 was considerable (150 species for example), this variation dimi- 

 nished. As, however, it would be very rash to assume that all 

 the miocene deposits of the United States, especially in countries 

 as far apart as Maryland and South Carolina, were of strictly con- 

 temporaneous origin, the fossil faunas of each region should be 

 carefully distinguished, and considered separately. 



Fossils of the Miocene Strata. 



Mollusca. — Mr. Conrad, in his work on the fossils of the ter- 

 tiary formations of the United States, Philadelphia, 1838, has 

 described and figured many of the miocene shells, but unfortunately 

 his work has never been completed. The same author has given 

 a catalogue of 187 shells of the middle tertiary, or miocene forma- 

 tion, in the Appendix to Dr. Morton's " Organic Remains " of the 

 Cretaceous Group. He has also described and figured some species 

 in Silliman's Journal, vol. xli. p. 344. In his own cabinet, he has 

 shown me more than 200 species of shells belonging to this forma- 

 tion, of which he considered about a sixth part to agree with 



