G3 



occurrence. Indeed, in some of the layers tliey are so numerous 

 as to be exposed to view in every mass of the sandy clay which 

 is broken or falls to pieces by its natural partings. Among 

 the most common of these fossils are Fusus quadricostatus, Pano- 

 pa\a refiexa, and many Mciocene species of Pecten, Area, Crassi- 

 tella, Cytherea, Venus, Astarte, Turritella, &c. In the lower 

 dark-colored stratum there occur stems of woody plants, which, 

 from microscopic indications, seem to have been coniferous, while 

 in the argillaceous layers both beneath and above, Prof. Rogers 

 has from time to time discovered prints of the leaves of Dicoty- 

 ledonous plants. 



In none of these deposits, whether of Eocene or Meiocene 

 age, do we meet with any remains of the shelly matter which at 

 one time must have formed so large a portion of the mass. In 

 various j^arts of the Tertiary region where the strata, as in the 

 present case, disclose only the casts, or as it were spectres of 

 shells, the sands and clays are more or less impregnated with 

 sulphuric acid, and present in some of their lower layers diffused 

 particles and even large and well-formed crystals of sulphate of 

 lime. We can therefore have little hesitation in referring the 

 disappearance of the shelly matter throughout this district to the 

 infiltration through these fossiliferous strata of water charged 

 with sulphuric acid, which, gradually transforming the carbonate 

 of lime into sulphate, carried this product downward to be ac- 

 cumulated and crystallized in the lower layers, at the same time 

 leaving the moulds and casts of the shells in so perfect a state as 

 to enable us to trace even their more delicate processes and 

 markings. 



The position of these beds along the western margin of the 

 Tertiary plain must be regarded as marking the general direction 

 of an ancient shore line ; and the frequent presence of fragments 

 of terrestrial plants in these Tertiary sediments is confirmatory 

 of this view. 



It has already been stated, in describing the section on Shockoe 

 Creek, that the lowest layer of the Tertiary is seen in that locality 

 resting upon a soft sandstone and conglomerate. This subjacent 

 deposit shows itself in a like position at several neighboring points, 

 and is evidently the thinning away of a formation, which, farther 

 toward the east, separates the Tertiary from the granite by a 



