43G N. ir. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



crystalline rocks to the sea, but northward it is confined to the higher 

 levels and is more widely eroded. 



POTOMAC FORMATION. 



Distribution. — The areal distribution of the Potomac formation in Virginia 

 and Maryland is represented on the map, and its characteristics and rela- 

 tions have been (described by McGee in his memoir on "Three formations 

 of the middle Atlantic slope." There are, however, a number of special 

 questions respecting its distribution, relations and stratigraphy which merit 

 brief discussion. 



Stratigraphy. — It has been suggested by McGee and held by Fontaine that 

 the argillaceous member of the formation is superior to the arenaceous 

 member and perhaps separated by a stratigraphic break. A study of this 

 question has, however, led me to believe that while the relative positions of 

 the two members are in the main as above stated, there is both a lateral and 

 vertical intergradation. In Virginia the arenaceous member occupies the 

 entire width of the outcrop belt. Along the Potomac river and northward 

 the argillaceous member is first seen at the top of the formation ; but by a 

 gradual lateral intergradation it appears to extend lower and lower in hori- 

 zon, finally to the exclusion of the arenaceous beds. Vertical intergradation 

 is well exposed in the Washington region, but evidence of the lateral change 

 is fragmental and less decisive. Northward from Washington to Baltimore 

 and beyond, the formation consists of clays, mainly of red and buff tints, with 

 intercalated sand streaks. This is the iron-bearing clay of Tyson, the varie- 

 gated clays of Fontaine, and the Baltimorean of Uhler. 



The evidence offered by Fontaine to prove the stratigraphic break in the 

 formation is in the clay belt at Hanover station and at Federal hill in Bal- 

 timore, where the variegated clays are underlain by sands. I have studied 

 these localities and many others of the same character, and have become 

 convinced that the sands are only lenses inclosed in the clay series ; a very 

 common occurrence in the Baltimore region. A boring made in the Federal 

 hill exposures passed through the sands into clays below, and in the Han- 

 over district and northward the clays are often exposed lying directly on 

 the crystalline rocks, both at high and at low levels. 



There can be little doubt that the Karitan clays of New Jersey are the 

 northern extension of the Potomac clays, for the outcrops are practically 

 continuous, the relations are precisely the same, and there is no evidence 

 of stratigraphic break, or overlap. 



The "Albinipean.'' — In 1888, Uhler* announced the discovery of a for- 

 mation lying unconformably between the Potomac and Severn formations, 

 for which the name "Albirupean " was suggested. 



* Proc. American Phil. Soc, vol. XXV, pp. 42-53. 



