184-6.] LYELL ON THE NEWER DEPOSITS OF NORTH AMERICA. 407 



brackish water. They are so placed as to show a vertical depres- 

 sion of the land to the amount of at least four feet. But a full ex- 

 planation of the curious appearances presented by them would re- 

 quire diagrams and a more detailed description than I have now 

 time to send. 



Thirdly, on the shores of the Bay of Mobile which opens into 

 the Gulf of Mexico, I first met with inland deposits of that bivalve 

 shell called Gnathodon cuneatus, an inhabitant of brackish water, 

 but now met with in banks of sand extending several miles above 

 the influence of the salt water, and rising three or four feet above 

 the height of the present tides. Mr. Conrad first directed my at- 

 tention to this remarkable formation, and I was taken to see it at 

 several places by the Rev. Mr. Hamilton at the mouth of the Ala- 

 bama river and in the suburbs of the city of Mobile. In neither 

 of these localities, where large and small individuals of the Gnatho- 

 don form dense, shelly masses, could I detect any intermixture of 

 other shells ; yet on exanuning the mud of the adjoining sea-shore 

 at low water, about a mile west of the embouchure of the Alabama, 

 I met not only with the Gnathodon abounding in a living state in 

 the mud, but also with a Nerita, and occasionally shells of Cyrena 

 carolinensis. I tasted the water here and found it perfectly fresh, 

 but was assured that this was owing to the wind blowing off the 

 shore, and that the same testacea cannot live in water perma- 

 nently fresh. The Gnathodon is not known to exist at present 

 at the mouth of any American river farther north than the Ala- 

 bama. The accumulation of fossils is far too considerable and ex- 

 tensive to be referred to the Indians, who are supposed by some 

 observers to have used them for food, and to have left the shells 

 scattered over the ground. On the other hand, the facts above 

 stated may perhaps be explained without assuming that there has 

 been an upheaval of the coast since the brackish waters of the Gulf 

 of Mexico were inhabited by the Gnathodon. It is possible that 

 the action of the waves on the shore may have thrown up these 

 shells in the form of sea-beaches. 



Fourthly, I have not fallen in with any deposits of miocenc shells 

 in the course of my present tour in Georgia and Alabama. The 

 most southern point where I know them to occur is South Carolina. 

 Dr. Gibbes of Charleston showed me a large number of miocenc 

 fossils obtained by him and Mr. Tuoniey on the banks of Goose 

 Creek, near the mouth of the Cooper river, at a place not visited by 

 me in 184-2. 



Fifthly, the eocene formations are of great thickness, and occupy 

 an extensive area in Georgia and Alabama. I formerly described 

 in the Society's ' Proceedings ' and * Journal ' the two formations of 

 this age which I saw in 1841-42 in South Carolina and Georgia, 

 the one chiefly calcareous, consisting of wiiite limestone and marl, 

 the other the burr-stone formation, composed j)rincipally of elay 

 and sand ; and I showed in a diagram reiirescnting a natural sec- 

 tion at ,Iaeks<)iil)()r(/, Georgia (.Toui'iial of the (ieol. Society, vt>l. i. 

 p. 4-.*^8), the manner in which th(se different nuMubers of the eocene 



