MARYLAND GEOLOGICAL SURVEY 105 



where a thin bed of drab clay carrying vegetable remains is overlain 

 abruptly with sands and gravels. Its contact with the Miocene is again 

 unfortunately obscure. At the localities just described no animal re- 

 mains have been discovered, but on the north bank of the Potomac, about 

 half way between St. Mary's Eiver and Breton Bay, there is a deposit of 

 lead-colored clay, exposed for a quarter of a mile along the shore. It is 

 buried at each end as well as above by sands and gravels and carries both 

 lignite and Rangia cuneata (Conrad). Although the description given 

 by Conrad is somewhat vague, it is highly probable that he visited this 

 locality and collected specimens of the fossils. Two more localities 

 still remain to be mentioned. Cornfield Harbor, and its companion de- 

 posit exposed 5| miles south of Cedar Point on the Bay shore. Conrad 

 was well acquainted with these deposits and to the former he devoted 

 special attention. Each is about 10 feet thick, occurs at the base of a 

 low cliff, is composed mostly of a dark, lead-colored clay, and is overlain 

 abruptly with Talbot sand and gravel, while unconformity on the Miocene 

 is beautifully shown at the base of the Bay shore section. A number of 

 fossils have been described from the Cornfield Harbor locality, among 

 which are Ostrea virginica Gmelin, Area ponderosa Say, Area trannversa 

 Say, Venus mercenaria Linne, Mya arenaria Linne, Barnea eostata 

 (Liune), Crepidula plana Say, Pohjnices duplieatus (Say), and Fulgur 

 carica (Gmelin). In this exposure the lower 4 feet of clay carries the 

 marine forms and above this there are 3 feet of sandy clay literally packed 

 with Ostrea virginica. These same general relations hold for the similar 

 deposits south of Cedar Point. 



The stratigraphic relation of these lenses of clay which are surely 

 unconformable on the underlying formation and apparently so with the 

 overlying sand and loams of the Talbot formation is a problem which 

 engaged the attention of the author until it appeared that the apparent 

 unconformity with the Talipot, although in a sense real, does not, how- 

 ever, represent an appreciable lapse of time and that therefore the clay 

 lenses are actually a part of that formation. In order to understand 

 more clearly what is believed to have taken place, these clay deposits 

 should be divided into two groups, those which carry plant remains con- 

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