30 



planicosta, a shell most characteristic of the Eocene period, both 

 in France and Great Britain, first suggested the relative age of 

 this deposit, which was confirmed by the fact that not one of the 

 species occurs in the Miocene beds near Charlotte Hall, twenty 

 miles distant from Piscataway. These remains are imbedded in 

 loose incoherent earth, a mixture of sand and clay. In the bank 

 of a contiguous stream, the matrix is 'hardened into a siliceous 

 rock similar to that in Georgia and Alabama. The surface of 

 this rock is constantly wet by springs which percolate through 

 the superincumbent stratum. 



At Fort Washington, on the Potomac, five miles from Piscat- 

 away, and on the road between the two places, I found the same 

 formation, consisting in some places of a friable, in others, of an 

 indurated fossiliferous marl of a dark gray color, replete with 

 the green grains of silicate of iron which are so characteristic of 

 the Cretaceous marls. It would appear that the Eocene strata 

 of Maryland, which rest immediately on the marls of the Creta- 

 ceous epoch, have been formed in part by the debris of the latter, 

 whilst ;the primary rocks have supplied the mica which is dissem- 

 inated through them in minute fragments. All the Eocene marls 

 effervesce strongly with acids. At Fort Washington, nearly all 

 the shells are decomposed except the Ostrece, which are usually 

 in a good state of preservation. A thick stratum of clay near the 

 fossiliferous deposits, forming the precipitous bank of the Poto- 

 mac river, contains abundance of Selenite, but the only trace of 

 organic remains I discovered, was a fragment of bone, the relic 

 of a marine animal. At the base, on the margin of the river, I 

 picked up a valve of Exoijyra, which led me suppose that the 

 Green Sand lies immediately beneath the surface, and probably 

 forms the bed of the river. 



The Cucullaea gigantea and Ostrea compressirostra are the most 

 abundant and characteristic fossils at Fort Washington. Some of 

 the small shells appear to be such species as occur at Claiborne, 

 but they are very imperfect and in most instances merely defined 

 by casts in tl;e coarse matrix. 



The Eocene occurs in Virginia, forming the western boundary 

 of the Pliocene, and will probably be indicated with tolerable ac- 

 curacy by the line drawn by Mr Maclure for the boundary of his 

 Alluvium, through Fredericksburg, Richmond, Petersburg, &c. 



In South Carolina, I have traced the formation in question, as 

 mentioned in the introduction, at Vance's Ferry on the Santee 



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