V 80 



this crust is a mixture of chloride, quartzose and micaceous sand, 

 resembling a variety of the secondary marl of Now Jersey. The 

 shells are numerous and too friable to collect, but they can be sa- 

 tisfactorily identified with those of Claiborne. At Piscataway and 

 Fort Washington, in the same county, where I first ascertained the 

 relation of these deposits to the London Clay, in 1830, the same 

 geological features are exhibited, and the superficial crust consists 

 of a fine bivalve, Panopea elomjata, nob. The Gryphcva vomer, 

 (Morton,) a fossil of the cretaceous strata and which closely resem- 

 bles Ostrea lateralis, Nillson, occurs not unfrequently in these 

 Eocene deposits. At Upper Marlborough, the Oxtrea compret&iros- 

 tra is very abundant and perfect and is washed out by rains from 

 the disintegrating superficial strata. It is remarkable that this is 

 the only Eocene fossil, which, after a careful exami nation of various 

 localities in Virginia and Maryland, I have detected in the medial 

 tertiary strata. In New Jersey, however a species common at 

 Claiborne is also found in a medial tertiary deposit, the Calyptrtva 

 t rock iform is , Lam . 



It would appear that the Eocene strata of Maryland, which rest 

 immediately on the marls of the Cretaceous epoch, have been form- 

 ed in part by the debris of the hitter, whilst the primary rocks have 

 supplied the mica which is disseminated through them in minute 

 fragments. At Fort Washington, nearly all the shells are decom- 

 posed except the Ostrea', which are usually in a good state of pre- 

 servation. A thick stratum of clay near the fossilif erous deposits, 

 forming the precipitous bank of the Potomac river, contains abun- 

 dance of Selenite,\)\it the only trace of organic remains I discover- 

 ed, was a fragment of bone, the relic of a marine animal. .At the 

 base, on tlfo margin of the river, I picked up a valve of Exoyyra, 

 which led me to suppose that the Green Sand lies immediately be- 

 neath Ihe surface, and probably forms the bed of the river, but as I 

 Afterwards found another valve near City Point in Virginia,it seems 

 probable that the species has been transported by currents which 

 washed the green sand of the Cretaceous epoch into the bed of the 

 Eocene sea. 



The Cuculhi'a yiyantea and Ostrea compressirostra are the most 

 abundant and characteristic fossils at Fort Washington. Most of 

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

 but they are very imperfect and in most instances merely defined 

 by casts in the coarse matrix. 



The Eocene occurs in Virginia, forming the western boundary 

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



[86] 



