JY. H. Darton — Magothy Formation of Maryland. 415 



bedded sands containing more or less brown sandstone. The 

 Potomac formation here is a densely packed, moderately fine, 

 " sharp " sand with more or less disseminated clay particles, a 

 member which is first seen in this vicinity and extends for 

 about twenty miles southwestward. This Potomac member 

 merits some special description here on account of its unusual 

 characters, relations to the Magothy formation and some mis- 

 apprehensions which exist regarding it. Its sands grade into 

 and give place to white and pink sandy clays at some localities 

 but they are usually characterized by their purity and extreme 

 compactness. At some points they are lithified in greater or 

 less part into white or gray sandstones or quartzites, and less 

 frequently brown sandstone. This sandstone constitutes the 

 " White Rocks " out in the Patapsco River near its mouth and 

 there are a number of other occurrences of it. This arena- 

 ceous Potomac member is, 1 suppose, the upper part of the 

 " Albirupean formation" of Uhler, but as it grades into typ- 

 ical Potomac clays and sands and is unquestionably not sepa- 

 rated by any stratigraphic break I see no grounds for its separa- 

 tion as a distinct formation.* 



On the neck between the Magothy and Patapsco Rivers the 

 sands and brown sandstones of the Magothy formation extend 

 inland for several miles covering the higher areas and capping 

 several small outlying knobs. 



Severn River region. — In the bluffs along the north shore of 

 the Severn River at Round Bay and for some distance above, 

 the Magothy formation and its relations are finely exposed. 

 Just south of the Round Bay hotel there is a bluff which with 

 the steep slopes above expose 140 feet of beds from the top of 

 the Magothy some distance up into the Pamunkey formation. 

 The Pamunkey beds here are weathered to brown, buff and 

 red sands with ferruginated masses, containing Eocene fossils. 

 Lying unconformably below the Pamunkey formation are 90 to 

 100 feet of the black micaceous, argillaceous and carbonaceous 

 sands of the Severn formation. Near the base of the bluff 

 the base of the Severn beds is exposed for several yards lying 

 unconformably on very coarse, white Magothy sands along an 

 east-dipping plane extending to a few feet above water level. 



In the next five hundred yards west these sands rise rapidly 

 and some low bluffs exhibit a thickness of twelve feet with a 



* The term Albirupean was proposed by P. R. Uhler in 1888 to the American 

 Philosophical Society (see Proceedings, vol. xxv, page 42). The fossils exhibited 

 and reputed to have been obtained from the " Albirupean " were shown by Heil- 

 prin and Lewis to be of Upper Silurian age and were probably fragments from 

 the Columbia formation. Its taxonomy was never clearly defined and its author 

 now includes under the name the greater part of the upper Potomac formation of 

 Maryland, all the sand lenses in the Potomac from top to bottom, apparently, and 

 I take it, the great Potomac series in Virginia. 



