62 THE EOCEXE DEPOSITS OF MAEYLAXD 



conitie, and of dark color, and is filled with shells or, more commonly, 

 casts of shells, among which the forms of Zone 2 are conspicuous, 

 together Avith Osirea compressirostra and TurriteUa mortoni in larger 

 numbers than in the lower beds. 



Zone Jf. — This zone is composed of a highly typical greensand, con- 

 taining in the main the forms mentioned above as occurring in zones 

 2 and 3. It is from 7 to 9 feet in thickness. 



Zone 5. — This limestone bed is very persistent and forms a conspicu- 

 ous ledge about 2 feet in thickness, along the face of the Aquia Creek 

 bluff until it passes below tide-water near its eastern extremity. In 

 addition to the species already mentioned as characteristic for Zones 2 

 to 4, which still remain common forms, there are two highly typical 

 species, tIz.: Pholadomya marylandica and Phenacomya petrosa, as well 

 as two or three gasteropods, Tudicla sp., Caricella sp., which from 

 the fact that they have only been found in the form of casts cannot be 

 further identified. 



Zone 6. — This thin layer, generally about 1 foot in thickness, of dark 

 characteristic greensand, is packed with the common forms of the pre- 

 vious beds. It thickens somewhat to the eastward along the face of 

 the bluff, and near Marlboro Point contains, among other forms, several 

 species of corals, including Eupsamniia elahorata, TurdinoUa acuticos- 

 tata, and Trocliocyatlius clarheanus. 



Zone 7. — The bed of greensand overlying the preceding layer is really 

 a continuation of it, although the fossils are few in number and much 

 broken. Fragments of the common forms of previous beds are found. 

 This bed is about 7 feet thick. It is barely possible that this stratum 

 of worn and broken shells represents an unconformity between the Pis- 

 cataway and Paspotansa members. Of this, however, there is no positive 

 evidence as yet, and we can only call attention to the fact that there are 

 physical indications of more disturbed conditions of sedimentation than 

 are usual in these formations at the very point where the faunal change 

 occurs. 



THE PASPOTAXSA MEMBER OR SUBSTAGE. 



The Paspotansa member, so-called from Paspotansa Creek, which 

 enters the Potomac river from the Virginia bank, a mile below Potomac 



