SiRAITGRAPTY OF THE POTOMAC GROUP 481 
stratigraphic relations of the higher portions of the Potomac 
deposits. 
It is the conclusion of the authors, founded upon a detailed 
stratigraphic study of the Potomac group, that all the beds 
which have afforded dicotyledonous types of plant life are above 
those which have yielded the vertebrate remains, and, more- 
over, that a marked unconformity exists between the two 
series of deposits. The evidence for this conclusion will be 
brought out in the succeeding pages. 
DESCRIPTION OF THE DEPOSITS. 
The several formations into which the larger unit of the 
Potomac group has been divided are as follows: 
(Raritan Formation) 
Lower Cretaceous - (Patapsco a ) Potomac 
Upper Jurassic (?) - ee * Group 
THE PATUXENT FORMATION. 
Name and areal distribution. —The Patuxent formation 
receives its name from the Patuxent River in the basin of which 
deposits of this horizon are found typically developed. As the 
basal member of the Potomac group the Patuxent formation 
occupies a position near the landward margin of the Coastal 
Plain, although the higher members of the series frequently 
overlap it and are found resting upon the crystalline rocks of the 
Piedmont Plateau to the westward. The Patuxent formation has 
been traced as a narrow, broken belt from Cecil county across 
Harford, Baltimore, Anne Arundel, and Prince George’s counties 
to the borders of the District of Columbia. 
Leading features of the deposits —The deposits of the Patuxent 
formation consist mainly of sand, at times quite pure and gritty, 
but generally containing a considerable amount of kaolinized 
feldspar, producing a clearly defined arkose. Clay balls are at 
times distributed in considerable numbers through the arena- 
ceous beds, which in places contain lenses of gravel, sometimes 
