548 [Feb. 26. 



The general elevation of the great plain does not exceed a hun- 

 dred feet, although it is sometimes considerably higher. Its width 

 in the middle and southern States is very commonly from 100 to 

 150 miles. The tide, except in the more southern States, flows 

 entirely across it, and the rivers intersecting it form large estua- 

 ries, which may have been due to the facility with which the in- 

 coherent materials of the cliffs were undermined and swept away, 

 a process of waste still going on. 



Throughout the greater part of the Atlantic plain, the cretaceous 

 rocks, if present, are concealed by the overlying tertiary deposits, 

 which consist chiefly of Miocene strata, extending from Delaware 

 Bay to the Cape Fear River, and occupying portions of Delaware, 

 Maiyland, Virginia, and N. Carolina, an area about 400 miles long 

 from north to south, and varying in breadth from 10 to 70 miles. > 

 There are, besides, some patches of the miocene formation in * 

 South Carolina and Georgia, where the Eocene, or older tertiaiy ' 

 deposits, predominate almost exclusively, and where the limits of 

 the miocene deposits have not yet been well ascertained. 



I have endeavoured to show, in a former paper, read to the 

 Society in February, 1843 *, that the fossils of Martha's Vineyard, 

 an island off the coast of Massachusetts, especially the teeth of sharks, 

 point to a near chronological connexion between the strata of that 

 island and the miocene strata of Europe ; but the evidence is far 

 more complete and satisfactory in favour of a similarity of age 

 between the deposits about to be described, which occur 350 miles 

 to the south, or in Maryland and Virginia. In the last-mentioned 

 States these formations extend over a wide area, and have been 

 well described by Mr. Conrad, who identified them in age with the 

 English crag, and called them " medial pliocene," and by Professors 

 W. B. and H. D. Rogers, in their article in the Philos. Trans, of 

 Philadelphia for 1836. These authors considered them entitled to 

 the appellation of Miocene, as containing a proportion of recent 

 shells corresponding to that known to characterise the miocene 

 deposits of Europe. 



The principal grounds which induce me to agree in this opinion, 

 and to regard the beds of sand, clay, and marl now under consi- 

 deration as corresponding in geological age with the crag of 

 Suffolk, the faluns of the Loire, and other contemporaneous 

 formations in Europe, are the following : — 



First. On the banks of the James River and elsewhere I saw 

 them resting on eocene deposits, or on sand and clay containing an 

 assemblage of shells resembling those of the London and Paris 

 basins. 



2dly. The genera of shells, and the amount of species by which 

 they are represented, agree for the most part with those which 

 characterise the European miocene beds. Many of the most abun- 

 dant species are allied, and some few are identical. 



* Proceed, of Geol. Soc, vol. iv. p. SI. 



