48 TRANSACTIONS OF THE [1889 



Creek. This is probably a member of the Middle Marl Bed, 

 and has thus far yielded no recognisable fossils. The northern 

 boundary of the Cretaceous, west of the Patuxent river, begins 

 in dark greensand beds only a few feet thick, resting in low 

 domes, at a distance of about a mile north of CoUington. From 

 that point it appears at irregular intervals in deep cuts of 

 streams and hillsides east of Buena Vista, and crossing the forks 

 of the western branch of the Patuxent, it is lost in the hills 

 north and east of Forestville. It is next found on Tinker's 

 Branch, and finally reaches the Potomac river in the high 

 tongue of land at Fort Washington. 



The bluffs running back from the river along the north shore 

 of Piscataway Creek being very steep, afford the best sections 

 to be obtained anywhere in the region west of the Severn river. 

 Here, however, we find only the two members of the Lower 

 Marl series, without the underlying thick stratum of sandstone 

 observed on the Severn, and destitute of indications of the Lamin- 

 ated Sands. A high cliff starts in front of the fort, and keeps 

 on with eroded intervals for a distance of fully three miles, and 

 then blends with the highlands further back. At the beginning 

 of this high tongue of land we observe the black Lower Marl 

 Bed at an elevation of twenty-eight to thirty feet above the 

 water, keeping on with almost unbroken continuity for about 

 three-quarters of a mile up the creek. After this, a bayed-out 

 space of more than a quarter of a mile in width cuts the stratum, 

 and it next appears as a boundary of this interval in the dome- 

 shaped end of a hill, from which the overlying greenish marl 

 has been denuded. Farther up the stream it may be seen at 

 intervals, but it is soon hid by the covering of later materials, 

 and is lost to view before the head of the open creek is reached. 

 Beneath Fort Washington, and at various intervals along the 

 cliffs of this side of Piscataway Creek, the Black Lower marl 

 stratum is about fifteen feet thick, and is hardened at base, or 

 changed into a thin broken layer of ironstone. The lower part 

 of the stratum also abounds in crystals of Selenite, and affords 

 casts of many kinds of shells in a soft state, coated with a brown 

 film of oxide of iron. The black stratum grades into the more 

 loamy greensand, which rises to an altitude of more than eighty 



