368 CONTRIBUTIONS TO THE GEOLOGY OF 



tion of these and the upper meiocene strata, and we infer that chemical 

 agencies of a like nature have operated upon both. 



Localities on the Potomac. 



Extensive and valuable exposures of the eocene are met with on this 

 river. These strata first show themselves a little above the mouth of 

 Acquia creek, and continue, with but few considerable interruptions, 

 as far as the eastern boundary of this deposit, at Mathias's Point. 



Throughout much of this distance a portion of the marl has the 

 character of a hard rock, of a yellowish white or greenish gray appear- 

 ance, abounding in shells and their impressions. The lighter coloured 

 variety is always more or less specked with green sand, in rather large 

 granules, and the darker contains this substance in larger quantity, 

 uniformly diffused throughout the mass. The material enclosing the 

 fossils, or their casts, consists largely of carbonate of lime, acting, ap- 

 parently, as a cement. This rock may therefore be regarded as an 

 eocene limestone. 



At a point about a quarter of a mile below the mouth of Acquia 

 creek, the cliflF, having a height of forty feet, exposes the following 

 strata : 



1 . From the water to the height of twelve feet is a yellowish gray 

 marl, specked with green sand, and abounding in shells, chiefly Cythe- 

 rea ovata (nobis) and CrassateUa (nobis). 



2. A ledge of rock, three feet in thickness, closely resembling the 

 marl in colour and composition. 



3. A layer of sandy clay, of a sulphur colour, containing shells, 

 principally Turritella Mortoni. This is five feet thick. 



4. A stratum of yellowish clay enclosing impressions of Turritella, 

 &c., and impregnated with the sulphates. This is twenty feet thick. 



About midway between the mouths of Acquia and Potomac creeks, 

 the bank has an elevation of about fourteen feet, and consists of ; 



1 . A layer of dark greenish blue marl, very remarkable for the mul- 

 titude of shells, principally CrassateUa (nobis), which it contains. 

 This rises only one foot above the water. 



2. A bed of shell rock, resembling the stratum beneath, but very 

 hard. One and a quarter feet thick. 



