1888] MARYLAND ACADEMY OF SCIENCES. 21 



Cardita planicosta may be found, and in some places are quite 

 numerous. 



Ascending the Potomac river, the cliifs composed of that 

 stratum still keep up to a nearly equal altitude until near the 

 mouth of Mattawoman Creek, where they recede from the shore 

 and are more or less hid in the denuded slope of the country 

 farther back. About two miles inland from the mouth of this 

 creek, low abrupt cliffs of the same stratum, twelve to twenty 

 feet in height, have been cut by erosion into the banks of the 

 stream, and in the dense marl, above the level of a man's head, 

 indurated casts of the OiicuUea gigantea are packed in great 

 numbers. Most of these specimens are destitute of the outer 

 shell, but yet, one in a considerable number is covered with 

 the shell altered to a chalky, more or less powdery condition. 

 This same blackish stratum reappears in the deep cuts made by 

 the Patuxent river, and extends along that stream from above 

 the village of Queen Anne to near Magruder's Ferry, where it 

 passes beneath the water and is lost to view. Like the Potomac, 

 this river has spread out in many places over the adjoining 

 country and denuded wide tracts between the hills, but at inter- 

 vals of every few miles, cliffs are seen rising thirty to fifty feet 

 above the water. Most of the region along the river, from 

 Priest's Bridge to above Hill's Landing, has been denuded, and 

 the surface is extensively covered by sand, gravel and loam so 

 as to hide the places where the best sections might otherwise 

 be observed ; but the ravines and deep gullies back from the 

 river show that the Eocene greensand is present above, and that 

 the micaceous marl beneath is undoubtedly a continuation of 

 the same as that in the adjacent rivers and creeks. The prin- 

 cipal difficulty in establishing locally the identity of the under- 

 lying strata in this region arises from the tender condition of 

 the casts of fossil shells. The presence of Ostrea compressi- 

 rostra, Cardita planicosta, Turritella mortoni, and Pectunculus 

 stamineus, in the ferruginous sandstone, as well as here and 

 there in indurated marl, shows conclusively that the Eocene 

 strata exist where these are found, as likewise in common with 

 the more abruptly prominent hills of the country farther 

 northeast. 



