192 



CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



Organic remains. — The flora of the Patuxent formation includes equi- 

 seta, ferns, cycads, conifers, monocotyledons and a very few archaic 

 dicotyledons, the coniferous and cycadean element being particularly 

 strong. The known fauna of the Patuxent formation is limited to a 

 single unio (Ward) and a fish (Fontaine). 



Strike, dip, and thickness. — The general strike of the Patuxent beds in 

 Maryland — and the same may be said of the other formations of the 

 Potomac group — is northeast-southwest. Strictly speaking, however, the 

 strike has toward the north a progressively more and more pronounced 

 eastward trend, ranging from north-northeast to south-southwest to the 

 southward of Washington to east-northeast to west-southwest at the head 

 of Chesapeake bay. A well defined change in strike occurs at the head 

 of the bay and another at or near Washington. 



The directions of the dip of the Patuxent, as well as of the overlying 

 beds of the Potomac group in Maryland, ranges from east-southeast to- 

 ward the south to south-southeast toward the north. The amount of the 

 normal dip of the basal beds of the formation reaches some 60 feet per 

 mile. Along the Fall line or zone, which is toward the landward mar- 

 gin of the Patuxent outcrop, the dip of the basal Patuxent beds is con^ 

 siderably greater than this. South of Washington it ranges from 50 to 

 75 feet. Near the Potomac river the descent reaches some 90 feet per 

 mile. The dip at other points is as follows : 



Feet. 



Burton sville 75 



Laurel 65 



Ilchester Hill 68 



Relay 200 



Catonsville 114 



House of Refuge 75 



Baltimore 80-90 



Towson 66 



Cub Hill 58 



Perry Hall 66 



Loreley 100 



Feet. 

 North of Joppa. 60-80 



Abingdon 100 



Harford Furnace. . 70 



Carsins 100 



Aberdeen 109 



Aldino 100 



Webster 100 



Havre de Grace. . . 100 



Battle Swamp 73 



Theodora 100 



Bay View 90 



Feet. 



Egg Hill 80 



Cherry Hill 60 



Barksdale 45 



Chestnut Hill 50 



Clifton Heights, Pa. 63 



Sand Hills, N. J.... 70 

 The basal Potomac 

 beds of the Raritan 

 River region, N. J. 



(Cook) 60 



From these facts we learn that along the zone of the Fall line the Pa- 

 tuxent deposits of the middle Atlantic border, barring two anomalous 

 exceptions, often exhibit a considerably steeper dip than 60 feet per mile, 

 the average for the section mentioned being 80 feet per mile; that this 

 value exhibits considerable variation and reaches its maximum in the 

 Patapsco depression at Relay at 200 feet per mile or more. 



The position of the Patuxent deposits of the Lutherville-Timonium 

 area is almost anomalous. Here the beds, which rest on crystalline 



