54 TEANSACTIONS OF THE [1889 



sand, diatoinaceous sand, and finally white plastic clay parted 

 by thin layers of yellow sandy clay, made up the chief part of 

 this division of the Cenozoic column. 



It does not seem likely that all of this series of strata formed a 

 superincumbent succession. In one place, as, for example, the high 

 cliffs near the mouth of Port Tobacco Creek, the acclivity shows 

 the blue-black marl at the base, rising thirty feet or more out 

 of the water, covered by a variable number of feet of whitish 

 sandy clay, becoming finer above, and grading upwards into 

 a bed of diatomaceous earth, varying from ten to forty feet in 

 thickness. The western side of the continuation of this combi- 

 nation, as it becomes exposed in the lower cliffs on the Potomac 

 river, above Smith's Point, presents a quite different body of 

 materials. It is there made up of highly fossiliferous black 

 plastic marl at base, from three to ten feet thick, overlaid by 

 paler, or more ferruginous somewhat sandy, less fossiliferous 

 marl, of variable thickness, which has been denuded on its 

 surface. Here not a vestige of the whitish sandy clay or of the 

 diatomaceous sand appears along any part of the section. At 

 this point a more distinct stratification is visible in the lower 

 bed, which also rises out of the water, and is not of the loose 

 texture seen on Port Tobacco Creek, but is very dense and 

 indurated. In this, too, we find incalculable numbers of fossil 

 shells, arranged in drifts or colonies. The lowest series, at and 

 below water line, being composed of friable shells of Dosiniopsis 

 meekii and D. lenticularis, with a mixture of Turritella mortoni 

 and a few small Ostreas. The next drift above this is com- 

 posed chiefly of CrassateUa capricranium, forming a series nearly 

 two feet thick, and enclosing a few specimens of the Dosiniopsis 

 and Turritella, with now and then an Ostrea. Above this, and 

 continuing up to near the surface, small specimens of the narrow 

 variety of Ostrea comp^essirostra form the chief contents of the 

 remainder of the bed. Here the strata thin out rapidly, so that 

 by the time that Smith's Point is reached the beds have 

 descended to below the water level and become lost to view. 

 As we proceed up the Potomac river these beds gradually rise 

 into higher cliffs, which on and above the Mattawoman Creek 

 reach a development only second to those which stand in monu- 

 mental prominence near the shores of Piscataway Creek. 



