REPORT ON THE GEOLOGY OF NORTH AMERICA. 33 



More extended researches in our recent conchology will 

 doubtless inform us, from time to time, of the existence in the 

 living state of some shells which we now regard as occurring 

 only in the extinct and fossil state ; and the natural tendency 

 will therefore be, to lessen Xhe apparent antiquity of each for- 

 mation. It is this prospect of being compelled to modify our 

 arrangement of the tertiary beds, as the researches multiply, 

 that has made me hesitate to fasten upon all of them the terms 

 of the new nomenclature, which they might otherwise claim. 

 I propose, therefore, to designate them for the present by the 

 convenient synonyms of 'newer tertiary' for the newer pleiocene, 

 * middle tertiary' for the older pleiocene and meiocene together, 

 and ' older tertiary' for the eocene. The newer pleiocene and the 

 eocene certainly exist in well-pronounced characters, and there 

 will be little or no necessity, now at least, to employ their pro- 

 posed synonyms ; but the case is very different with the less de- 

 fined formations of an intermediate age, and I shall therefore find 

 it of essential assistance to employ, for these, the term ' middle 

 tertiary'. American geologists will be careful not to confound 

 the middle tertiary beds, of which I am speaking, with those 

 which Mr. Conrad has designated by the same name, and which 

 are clearly of eocene date, as both that gentleman (in his re- 

 searches among the fossils of Clairborne, Alabama,) and Mr. 

 Lea have been prompt to show. 



Newer Pleioce?ie of St. Mary's County, Maryland. — In the 

 tertiary mass now before us, the number of well-characterized 

 shells is such as to enable us to examine their relations to the 

 species now living in the neighbouring ocean, or peculiar to 

 other formations. For our knowledge of the formation at the 

 mouth of the Potomac, we are indebted exclusively to the re- 

 searches of Mr. Conrad ; and it is mainly from his descriptions 

 and on other information which he has been kind enough to im- 

 part, that I am enabled to present the following brief account 

 of the deposit. He justly pointed out its very modern character, 

 by showing the identity of nearly all the species with the shells 

 at present living on our coast. Mr. Conrad thus describes the 

 formation : 



"About three miles north of the low sandy point which forms 

 the southern extremity of the peninsula, the bank of the Poto- 

 mac rises to an elevation of about fifteen feet at its highest 

 point : the fossils are visible in this bank to the extent of a 

 quarter of a mile. The inferior stratum is a lead-coloured clay, 

 containing vast numbers of the Mactra lateralis of Say, which 

 in many instances appear in nearly vertical veins, as though 

 they had fallen into lissures. The Pholas costata is also nu- 



1834. D 



