58 TRANSACTIONS OF THE [1889 



It seems quite likely that this body of intervening mixed 

 material is not in its original order of deposition. The Lati- 

 area gigantea, which elsewhere occupies the lower twenty feet of 

 the black marl, here may be found buried in a pile of broken 

 shelly matter, at various altitudes, reaching up to fifty feet 

 above the bed of the branch. 



This example will serve as a type of the ravines which are 

 found on the beds of the various deep-cutting brooks that cross 

 Pomonkey Neck, or enter the Piscataway west of the village of 

 Old Farmington. These enormous deposits of fossil shells 

 belong to the region lying west of a north-northeast by south- 

 southwest meridian passing near the middle of Prince George's 

 and Charles counties. East of the Patuxent river the Eocene 

 fossiliferous strata take on a different fades, are subdivided into 

 several members, and lack, with perhaps only two or three small 

 exceptions, the compact black marls which are so immensely 

 developed near the Potomac river. 



No sharply defined vertical limits can be given for the species 

 or genera of the fossil shells in the strata recorded above. In 

 the black marl, which seems to be the fossiliferous member most 

 distinctly in place, the Ostrea coiwpressirostra occupies the 

 lowest position, and it occurs in company with Latiarca gigantea 

 up to twenty feet above the base of this bed. Above this level, 

 and sometimes scattered below it, Crassatella ijrotexta, C. alae- 

 formis, and Dosiniopsis meekii appear in their greatest abun- 

 dance, the Dosiniopsis coming in more particularly above the 

 others and extending up to a height of about twenty-five to 

 thirty feet above the base. Above this stage occurs Turritella 

 mortoni, accompanied by a few small specimens of Ostrea com- 

 jpressirostra, and an occasional individual of the Dosiniopsis, 

 Glycimeris elongata, Cardita planicosta, Latiarca idonea, and 

 Latiarca ononchela. The Turritellas sometimes form a bed by 

 themselves, a foot or more in thickness, with an interval of one 

 or two feet before the next overlying stratum of fossils comes 

 into view. The uppermost stratum is usually composed of 

 more or less packed specimens of the same Ostrea, which often 

 make a collection from two to five feet in thickness, and which 

 appear on the surface wherever denudation has cut off the super- 



