THE TERTIARY FORMATIONS OF VIRGINIA. 369 



3. A layer of yellow sandy clay, containing Turritella Morioni, and 

 other shells. This is at least seven feet in thickness, and is capped by 

 a thin stratum of yellow clay. 



In proceeding downwards, the shell rock, dipping gently to the east, 

 becomes lower in the bank, and at length disappears near the mouth 

 of Potomac creek. The bluish marl continuing beneath, first passes 

 out of view. 



The greenish blue marl again comes in view at the landing on the 

 south side of Potomac creek, and still further down, at about half a 

 mile below the mouth of Paspitansy creek, the bank of the Potomac 

 presents : 



1. A stratum of this dark coloured marl, seven feet thick, contain- 

 ing some green sand and numerous shells, chiefly Turritella Mortoni, 

 Crassatella (nobis) and Ostrea. 



Q. A bed of yellowish and reddish clay, thirteen feet thick. 



A little below this point a ledge of the shell rock makes its appear- 

 ance in the bank, and continues, with but little interruption, down the 

 river for several miles. 



At some points, two of these ledges are seen, one near the water 

 level, and one at a considerable height on the face of the bank. This 

 rock is replete with fossils and their casts, and consists, in large part, 

 of carbonate of lime. In the same bank the marl is seen in the softer 

 condition, and of both the yellowish and greenish blue varieties, over- 

 laid by a stratum of the gypseous and acid clay. Among the interest- 

 ing fossils here found, are two beautiful species of Cucullxa. 



At the Eagle's Nest and Mount Stuart, about three miles above 

 Boyd's Hole, the eocene strata are well exposed for some distance along 

 the river bank. 



At the former locality, the banks, which are from twenty to twen- 

 ty-five feet in height, are composed of two strata, the lower, which is 

 about twelve feet thick, consisting of dark bluish clay and sand strongly 

 imbued with copperas and containing a little gypsum, and the upper 

 of coarse ferruginous sand and gravel. A few hundred yards below 

 this point a thin layer, containing fossils, comes in view, about midway 

 between the top and bottom of the bank, and as we proceed down the 

 river this shelly stratum expands in thickness, its upper boundary con- 



