MAIJYLAXD GEOLOGICAL SURVEY 57 



Glauconite sokloin, if ever, occurs pure in nature, but is mixed with 

 greater or less amounts of arenaceous materials, ])ro(hHin<: what is 

 known as greensand, a term whicli is commonly made to emljrace the 

 argillaceous deposits as well, particularly when the glauconite grains 

 are visible, although they are more correctly green clays. When the 

 deposits are distinctly calcareous, they are generally known as green- 

 sand marls. Xo definite percentage of any of the constituents is re- 

 quired, and as they are so commonly intermingled the terms just des- 

 cribed are used somewhat indiscriminately. 



DEPTH OF SEDIMEXTATIOX. 



The depth at which sedimentation, as shown by the existing Eocene 

 strata, took place cannot be definitely determined, but the character 

 of both the deposits and the fauna points to seas of moderate depth, 

 probably from 100 to 300 fathoms in the southern portion of the area; 

 while the coarser and less giauconitic materials to the northward sug- 

 gest even shallower depths. Bagg reports the foraminifera to be mainly 

 of shallow water types while the other classes of organisms are chiefly of 

 the same character. 



The giauconitic materials of which the formations are so largely com- 

 posed show that sedimentation must have been slow, but whether this 

 was due to the fact that the deposition went on far from the shore-line 

 or to the fact that the rivers were draining a surface approaching base- 

 level cannot be satisfactorily determined, although recent work on the 

 physical history of the Appalachian region since the Cretaceous points 

 to the latter explanation as probable. 



Stratigeaphical axd Paleoxtological Chaeacteeistics. 

 The Eocene deposits of Maryland and adjacent states were described in 

 1891 by Mr. X. H. Darton ' of the U. S. Geological Survey under the 

 name of the Pamiud-ey formation. More detailed investigations by the 

 authors of this report indicate that sufficient lithologic and paleonto- 

 logic differences exist to warrant the establishment of two formational 

 units instead of one. The presence of an argillaceous bed at the base 



1 Bull. Geol. Soc. Amer., toI. ii, 1891, p. 411. 



