72 TEANSACTIONS OF THE [1890 



of the long tributary of Beaver Dam Branch. The same fea- 

 tures likewise recur in the hills opened by other tributaries of 

 this same branch, about one mile north of Central avenue. At 

 Seat Pleasant Post Office, about half a mile west of this larger 

 territory of Beaver Dam Branch, the shell-marl has been reached 

 at a depth of sixteen feet from the surface, in a well dug at the 

 summit of the hill. After leaving this section the shell-marl of 

 this same type occurs in ravines of nearly all the branches 

 emptying into the West Branch of the Patuxent, and continues 

 on beyond Upper Marlboro. The fossil shells taken from these 

 exposures of the marl, as far as it has been examined for them, 

 are Turritella mortoni, T. humerosa, Ostrea compressirostra, 

 Dosiniopsis meekii, Nuculana protexa, and Crassatella capri- 

 cranium. Fragments of shells are densely packed in many 

 parts of these marls, and no doubt a close examination would 

 reveal many more species than the six enumerated above. The 

 material corresponds very closely with that which holds La- 

 tiarea gigantea, and Cardita planicosta in the region of Piscat- 

 away creek, but the marl in the vicinity of the District of Co- 

 lumbia has thus far failed to yield either of those species. 



The upper, or ferruginous, stratum of the Eocene appears to 

 have but a very restricted exposure in this part of the country. 

 Fragments of the iron-brown sandrock containing casts of the 

 Eocene shells have been picked up on the surface or in the 

 ferruginous soil of the hills south of the District of Columbia. 

 It is only, however, after we have passed to a distance of about 

 three miles from the above named line that the superficial shell- 

 rock forms a distinct layer in place. The best locality for this 

 exposure known to me is that directly at the eastern end of 

 Central avenue, on Dr. Berry's farm, where, at a level of six or 

 more feet above the road, the rock forms a broken stratum 

 about one foot in thickness. It crops out along the brow of a 

 low hill there, is moderately ferruginated, and is densely packed 

 with casts, and less frequently with the tests, of the shells. 

 In this stratum, as indeed in places detached from it, the 

 Cardita planicosta is accompanied by both species of Turri- 

 tella, Ostrea compressirostra, and an assortment of the other well- 

 known shells. 



Several sections are available for illustrating these obser- 

 vations, but there is not space for them at this time. 



