66 TRANSACTIONS OF THE [1889 



38. T. humerosa Con. Generally distributed. 



39. T. sp? Queen Anne. 



40. Architectoniea sp ? Piscataway. 



41. Natica interna Say. Piscataway. 



43. N". sp ? Piscataway, etc. 



48. Lunatia eminula Con. Piscataway. 



44. Infuudibulum ? sp ? Near Annapolis. 



45. Volutilithes sp ? Piscataway. 



46. V. sp ? Queen Anne. 



47. Athleta Tuomeyi? Con. Piscataway. 



48. Triton Mississippiensis Con. Piscataway. 



49. T. sp? Piscataway. 



50. Fusussp? Piscataway. 



With reference to preceding observations, and regarding the 

 levels at which the Eocene fossils occur, it becomes evident that 

 these fossil remains have not always been deposited in the same 

 manner. It has already been noticed how in the strata of black 

 marl the species of shells rest in elongated horizontal drifts or 

 colonies, often composed of a single kind, while in the various 

 types of greensand marls and marly sands they are thrown 

 together in confused piles, or distributed in thin sheets at 

 irregular intervals. It is, however, satisfactorily clear that in 

 most cases the beds and their contents still occupy the places in 

 which they were originally laid down. This may be said 

 especially of the black compact marl strata which extend all the 

 way from Tinker's Creek, in Prince George's county, to Smith's 

 Point, on the Potomac side of Charles county. Farther inland, 

 on the shores of Port Tobacco Creek, and down the Potomac to 

 near Ludlow's Ferry, the high bluffs of black marl are more 

 sandy, and the component materials, being more acrid, have 

 eaten away the shells from the fossils and reduced them to mere 

 friable casts, coated with a stain of iron-rust. 



But right alongside of the black marls in Prince George's 

 county, the thick deposits of calcareous and siliceo-calcareous 

 materials, charged with greensand and comminuted shells, form 

 a heterogeneous mixture, which holds strata of silicious rock 

 packed with fossils. Proceeding eastward, these beds grow 

 thinner, and become deep-seated in the body of the hills. So 

 that by the time Upper Marlboro is reached, the mixed material, 

 composed in great part of fragmentary calcareous shells, has 



