REPORT ON THE GEOLOGY OP NORTH AMERICA. 39 



2nd, blue marl, full of Venus, twelve inches ; 3rd, blueish 

 green marl, four feet thick, having at the bottom a mass of 

 Pecten, and belovr this a crowded layer of Perna, all perfectly 

 horizontal. 



On the York river the stratification, though it does not ex- 

 hibit so lofty a precipice fiUed with shells as before described 

 on the James river, presents the clay strata very beautifully. 

 Immediately overlying the shells is a continuous bed of clay, 

 many miles long, in some places forming a vertical wall, ten or 

 fifteen feet high, and as smooth as masonry. It is of a blue 

 colour, and divided by thin layers of sand, perfectly horizontal, 

 into portions about eight to twelve inches thick, so that the ap- 

 pearance is very much like that of a wall. The incumbent clay 

 in some places thins out, and changes colour to a reddish brown, 

 which makes it scarcely distinguishable from the diluvium above; 

 sometimes it is subdivided into two strata, separated by sand 

 and gravel. This clay is a very common deposit throughout 

 the tertiary marl region, sometimes beneath, sometimes above 

 the shells, and often both below and above, and also containing 

 shells. In appearance it resembles a clay which is a member of 

 the secondary greensand formation of New Jersey. From York 

 town, six miles up the river, is the following interesting section : 

 1st. Near York, a curious rock, containing shells, often in minute 

 fragments, being somewhat like masses of the crag of England. 

 Here the strata are not horizontal ; but in a ravine below the 

 town they dip on opposite sides toAvards the ravine at an angle 

 of more than thirty degrees. This shell rock is an indurated cal- 

 careous sand, formed of shells, not partially decomposed, but 

 comminuted by attrition. It had obviously been subjected, as 

 Mr. Conrad observes, to a violent action of the waters at a pe- 

 riod anterior to the tranquil deposition of the perfect shells it 

 contains. 2nd. From York town to about three miles up the 

 river, the principal stratum consists of shells, overlaid by the 

 above-mentioned blue clay, separated near its western end into 

 two strata. 3rd. At some distance higher up, the shell rock 

 again takes the place both of the stratum of shells and the over- 

 lying blue clay. 4th. Beyond this again, and still on the same 

 level, the blue clay is seen resting once more on the unbroken 

 shells. The appearance of the wall of clay in these places is 

 very curious ; it is as smooth as if cut with a spade, and re- 

 sembles the wall of a fortress. On the Nansemond river, in the 

 immediate neighbourhood of Suffolk, very nearly the same series 

 of strata is seen as described upon the York and James rivers. 

 Yellowish sand reposes most generally upon the blue clay, both 

 beds containing a profusion of shells, and rising from the rivei 



