Association of American Geologists and Naturalists. 255 



ever, that the districts which have led Mr. Conrad to an opposite con- 

 clusion, viz, the vicinity of Claiborne in Alabama, and of Upper Marl- 

 borough in Maryland, w^ere not visited by Mr. Lyell, and it is possible 

 that, had he inspected the whole ground, he might have somewhat modi- 

 fied his opinion. Nevertheless, the evidence presented by Mr. Conrad 

 does not seem entirely convincing, since his account of the conditions 

 under which the Gryphea vomer occurs in Maryland and the Plagios- 

 toma dumosum was found at Claiborne, do permit the inference that 

 they were swept by currents into the eocene waters from an adjacent 

 upraised cretaceous deposit. Even if it should be proved that these fos- 

 sils lived associated with the earlier tertiary races, so preponderating are 

 the true eocene forms, both as respects variety and abundance, that it 

 would still seem inexpedient to class the stratum containing the mixture 

 as a transition bed between the secondary and tertiary. Would it not 

 be more philosophical indeed to suppose that the two or three intruding 

 races had escaped the general catastrophe which cut off all the rest of 

 the larger cretaceous species ? 



Mr. Lyell, I am gratified to perceive, fully sanctions the application 

 of the terms eocene, miocene and pleiocene, to the respective divisions 

 of our Atlantic tertiary, as made by me in 1834, from data derived 

 chiefly from the palseontological determinations of Mr. Conrad, and this 

 latter gentleman now lends the weight of his valuable authority to the 

 correctness of the generalization. 



The proportion of living to extinct species in our eocene strata appears 

 to be as minute as in the corresponding beds of Europe, amounting 

 probably to not inore than one or two per cent., there being about two 

 hundred and fifty species definitely ascertained. The average ratio in 

 our miocene is about that in the Faluns of Touraine, Mr. Conrad having 

 identified as living species thirty eight in a list of two hundred and thirty 

 eight at present known to him. 



The interesting exhibition of tertiary strata at Gay Head in Martha's 

 Vineyard, referred by Prof. Hitchcock to the eocene period, and by some 

 conjectured to contain fossils washed from a cretaceous formation, has 

 been examined by Mr. Lyell, and is pronounced by him to appertain to 

 the miocene age. It has yielded him some interesting organic remains, 

 none of which are of eocene genera, while several of them are identi- 

 cal with species characteristic of the miocene beds of Maryland and 

 Virginia. 



In alluding to the more interesting general determinations connected 

 with the tertiary strata of the United States, I ought not to pass over the 



