368 W. B. Clark — Potomac River Section 



common species of the previous zones. In the main portion of the 

 Aquia Creek bluff this bed is only one foot in thickness, but 

 thickens to the eastward, and just above Marlborough Pt. contains 

 among other forms several species of corals, including Eujpsam- 

 mia elaborata, Turbinolia acuticostata, and Paracyathus (?) 

 clarheanus. 



No. 7. Overlying the preceding bed, and really a continua- 

 tion of it, is a zone in which the fossils are few in number and 

 much broken. This bed is about seven feet in thickness. 



No. 8. The highly characteristic green-sands and green-sand 

 marls of the previous zones aVe succeeded by a bed some thirty 

 feet in thickness, in which the glauconitic grains have been exten- 

 sively weathered, giving the strata a greenish-grey appearance 

 which changes to a reddish-brown in the upper layers. Several 

 irregular bands packed with Turritella mortoni are present both 

 in the Aquia Creek and Potomac Creek sections, while associated 

 with that species at both localities are Turritella humerosa, 

 Cucullwa gigantea, Crassatella alwformis, Ostrea compressi- 

 rostra and other forms. The upper portions of this bed have 

 afforded most of the species from the Potomac Creek bluff. 

 This zone forms the base of that bluff, while it is more than 

 thirty feet above water level in the Aquia Creek section three 

 miles above. 



No. 9. The thick-bedded limestone layers which compose 

 this zone are almost exclusively made up of the shells of Turri- 

 tella mortoni, forming a true Turritella rock. Between the in- 

 durated bands are layers of unconsolidated and much-weathered 

 green-sand which contain very few fossils of any description. 

 Great masses of this Turritella rock strew the shore at the 

 base of both the Aquia Creek and Potomac Creek bluffs. The 

 bed is about ten feet in thickness in the Aquia Creek bluff, but 

 reaches fourteen feet in places in the Potomac Creek section. 



No. 10. The greenish-grey sand overlying the Turritella bed 

 is more argillaceous than the underlying beds of the Eocene. 

 The glauconite grains have been much weathered and nearly 

 all trace of the shell substance removed from the few forms 

 recognized. The casts found at the Potomac Creek bluff are 

 chiefly those of a Gytherea. No fossils were found at the 

 Aquia Creek bluff. The bed is about twenty-five feet in 

 thickness. 



No. 11. A thin highly indurated layer of argillaceous green- 

 sand overlies No. 10 in the Potomac Creek bluff, and among 

 several indeterminable casts a few specimens of Venericardia 

 planicosta were found. 



No. 12. This bed of greenish-grey argillaceous sand still 

 shows some unweathered grains of glauconite, but is devoid of 

 fossils so far as observed. It reaches eight feet in thickness. 



