202 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



Sometimes the Raritan sands are cemented by silica, producing a 

 highly resistant rock resembling quartzite. An illustration of this occurs 

 at the White rocks of the Patapsco river which afforded the name " Albi- 

 rupean " applied by professor Uhler to the upper portions of the Potomac 

 group (plate 27, figure 1). 



The clays are commonly of light color or white. Sandy, white clays 

 occur on a large scale. These, like the fine arkosic sands, are locally 

 known as " fuller's earth " (plate 27, figure 2). At times the clays be- 

 come dark colored, lignitic, very slightly iron-bearing, and richly leaf- 

 bearing. They maybe either laminated or less frequently massive, and 

 at times exhibit a conchoidal fracture. A characteristic bluish drab tint, 

 with a tendenc}^ toward lamination, serves to distinguish them, as a rule, 

 from the drab Patapsco clays. Variegated or " pied " clays greatly re- 

 sembling the " variegated clays " proper of the Patapsco formation also 

 occur, but they are commonly more sandy and of somewhat lighter tints, 

 often pinks, being known to the northward as " peach-blossom : ' clays. 

 The Raritan variegated clays are also apt to exhibit obscure stratifica- 

 tion and were probably redeposited from the Patapsco. The scale upon 

 which they occur is inconsiderable as compared with those of the Patapsco, 

 and the formation is on the whole considerably less argillaceous and also 

 less homogeneous. 



Strike, dip, and thickness. — The strike and dip of the Raritan formation 

 in Maryland correspond, in a general way, with those of the preceding 

 terranes. The normal dip of the basal beds of the formation is about 30 

 feet per mile. When the deposits extend landward as far as the Fall line, 

 as in Cecil county, there are well marked increases in dip. An artesian 

 well boring at Rock Hall, on the Eastern shore of Maryland, encoun- 

 tered the Raritan beds at 240 feet below tide level, indicating a dip of 

 34 feet per mile for the deposits of central Maryland. 



The thickness of the Raritan formation in central Mayland, along the 

 line where its upper beds descend below tide, is estimated at 240 feet. 

 The greatest thickness of the exposed beds in a single section occurs at 

 Maulden mountain on the west shore of Elk neck where it reaches 

 nearly 70 feet (plate 27, figure 2). 



Beds of black, massive, pyritous, earthy lignite, bearing prostrate 

 trunks of lignitized conifers, honeycombed by Teredo, and associated 

 with layers of comminuted lignite, occur near the summit of the forma- 

 tion. Overlying these are beds of coarse and fine, often crossbedded, 

 slightly lignitic, white and buff sands with interlaminated brown sandy 

 loam. These are often indurated. This entire series, which is best ex- 

 posed at cape Sable, on the Magothy river, comprises the so-called u alter- 

 nate clay-sands " of Uhler and the " Magothy formation " of Darton. 



