THE TERTIARY FORMATIONS OF YIRGINIA. 359 



feet, a stratum of blue sandy clay, containing impressions of shells of 

 several different kinds, among which the Pecteti Madisonius, Venus 

 mercenaria^ Venus cortinaria (nobis^, and Mactra modicelia are the 

 most frequent. Upon the face of this clay, especially where it pro- 

 jects from the general cliff, a copious efflorescence of sulphate of iron 

 is usually found, imparting a greenish yellow colour to the surface. 

 At other more retiring parts of the cliff, a white, and somewhat gra- 

 nular coating of sulphate of lime, is equally abundant, and small silken 

 crystals of this substance are generally disseminated through the ma- 

 terials of the stratum. On the surface of this bed delicate crystals of 

 sulphate of magnesia may likewise be discerned. This stratum is 

 overlaid by a band of indurated ferruginous clay, approaching to the 

 hardness of rock, and filled with a material closely resembling pipe- 

 stem ore. This is about two feet thick. Next above is a stratum 

 consisting alternately of sand and ferruginous mottled clay, extending 

 to a height of about forty feet ; and lastly, is a layer of diluvial gravel, 

 covered with a shallow soil. 



Further up the river the cliffs attain a greater elevation, being in 

 some places about one hundred feet in height. Here the same strata 

 occur, and in the same order as before. Proceeding still higher up 

 the river, a band of shells makes its appearance upon the face of the 

 cliff, at a height of about fifteen feet above the water. This rises, as 

 we ascend the river, with a gentle inclination, until at its northern ex- 

 tremity it is fifty or sixty feet above the beach. The width of this 

 band is about five feet, and its length, though not without occasional 

 interruptions, about one and a half miles. 



The material of this stratum is a bluish sandy clay, very similar to 

 that before described, but containing no appreciable amount of the va- 

 rious sulphates observed to be present in the former. The shells are 

 very numerous and perfect. Among them are vast numbers of the 

 Perna maxiUata, of small size, as well as Turritella plebeia, Mactra 

 modicelia, &c., with an occasional ^rca idonea, and other larger 

 shells. Above this bed is a heavy stratum of clay, of a mottled ap- 

 pearance, and higher still, and distant about twenty feet from the for- 

 mer, a second fossiliferous layer, of a lighter colour, and containing 

 fewer shells. 



YI. — 4 p 



