352 CONTRIBUTIONS TO THE GEOLOGY OF 



banks rise to twenty or twenty-five feet, and consist of yellowish and 

 reddish clays, containing no trace of the green sand or fossils of the 

 eocene. Still further down, the cliflfs are replaced by a low and re- 

 tiring shore, beyond which the beds of meiocene marl first come in 

 view. 



It is obvious from these details, that the eastern limit of the eocene 

 is marked on both rivers by the occurrence of a region of like geologi- 

 cal and topographical features, immediately east of it, and by great simi- 

 rity in the arrangement and composition of the contiguous strata. As on 

 the James and Pamunky rivers, as well as in the district of which 

 we are now treating, the eocene is skirted on the east by a level and 

 comparatively low district, comprising only beds of sand and clay, des- 

 titute of fossils. It would seem a probable conclusion that these bar- 

 ren strata mark the period of disturbance W'hich terminated the epoch 

 of the eocene deposits, a period attended with such important changes 

 in the condition of the neighbouring seas, as to destroy all, or nearly 

 all, the species of shell fish then inhabiting them, and to adapt their 

 waters to that multitude of new species which were brought to light 

 in the succeeding epoch of the meiocene. 



Between the two points thus fixed upon as the extremities of the 

 eastern boundary of the eocene in the neck, several intermediate locali- 

 ties have been marked, but from the obscurity of the exposures, no 

 very certain indication could be derived as to the precise figure of the 

 boundary line, in the intervening space. There is little doubt, how- 

 ever, that it will be found to depart but in a very slight degree from 

 the right line connecting the two points above described. 



The western boundary of the eocene remains next to be described. 

 In drawing the line of demarcation here, as in the former case, a few 

 well determined points are relied upon for fixing its general direction, 

 and the intervening irregularities are not attempted to be laid down. 

 Indeed the absence of any satisfactory exposures of the strata, through- 

 out a distance sometimes of several miles, renders this the only method 

 of proceeding at present practicable. 



The guiding points, in fixing the western limits of the eocene in the 

 peninsula, are : 



Firsts The mouth of Massaponax river, in Spotsylvania county. 



