20 TRANSACTIONS OF THE [1888 



surface. In the lower part of this upper series of beds, at a 

 considerable elevation above the river (perhaps as much as sixty 

 feet), the indurated sandrock forms a broken uneven vein, in 

 which Turritella mortoni and obscure impressions of other shells 

 occur more or less abundantly. In the firm more clayey part 

 of these beds, at a higher level, as well as in the ferruginated 

 portion nearer the surface, numerous hard casts of the large 

 CucuUea gigantea, with more or less of the shell attached, are 

 found accompanied by the largest of our fossil oysters, the 

 Ostreoj Gompressirostra Say. This last species is also common 

 in the soil of the ravines near their summits, and specimens 

 may be picked up in similar situations at many points along 

 the Piscataway Creek and back into the country of Pomonkey 

 Neck. South of this district the dark blue-black Eocene marls 

 rise into hills of various altitudes, and are cut into abrupt 

 cliffs along the greater part of the Potomac side of Charles 

 county to near Ludlow's Ferry, a short distance above Lower 

 Cedar Point. This marl closely resembles that of the Creta- 

 ceous Lower Marl Bed in color and general appearance, but it 

 is less micaceous, not flaky, more aluminous, and composed of 

 finer sand in its upper portion. It grades upwards into pale 

 drab clayey sand, which becomes whiter as it ascends until the 

 stratum of Tripoli, or diatomaceous earth, is reached at the 

 base of the Miocene formation. The dark layer reaches an 

 altitude of thirty feet a short distance above the mouth of Port 

 Tobacco Creek, while the paler overlying portion adds ten feet 

 or more to its thickness in the same region. 



Numerous well-known Eocene shells occur in these bluish- 

 black marls, and in the ravines which penetrate the hills back 

 of the Pomonkey ; and farther down to near the Mattawoman 

 Creek, beautifully preserved fossil shells may be dug out of the 

 beds. Some of these, such as Cytherea ovata and Dosiniopsis 

 meeJdi, are so well preserved that the nacreous color of the shell 

 still remains. In these beds we find also Orassatella capricra- 

 nium, Gucullea transversa, Orassatella alta, Cardita planicosta, 

 Glycimeris elongata, Nuculana protexta, and Pectunculus stami- 

 neus. In the more sandy portions of the bed, especially in its 

 upper portions, Turritella mortoni, Ostrea Gompressirostra, and 



