STRATIGRAPHY OF THE POTOMAC GROUP 489 
Many other characteristic local sections might be given, 
since the Arundel formation has been opened at numerous 
points for iron ore. Lenses have been observed among other 
places at the head of Elk River neck in Cecil county, in 
the vicinity of Joppa, Harford county, on Stemmers Run, 
Baltimore county, at Locust Point, Baltimore city, and at 
numerous localities in Anne Arundel and Prince George’s 
counties. 
« Fossils —Animal and plant remains have been observed at 
several localities in the Arundel formation. The Muirkirk area 
has afforded much the largest number. It was in this section 
that the vertebrate and cycadean remains of the Potomac group 
were first discovered by Mr. Philip T. Tyson. Later Professor 
O. C. Marsh of New Haven made extensive collections at this 
locality and upon this material based his conclusions regarding 
the age of the Potomac group. The vertebrate fossils consist 
largely of Dinosauria. 
The plant fossils consist of ferns, conifers and mono- 
cotyledons. No dicotyledonous forms have yet been recog- 
nized. 
THE PATAPSCO FORMATION. 
Name and areal distribution. — The Patapsco formation is 
so called from its typical occurrence in the valley of the 
Patapsco River. It extends entirely across the state from the 
Delaware border to the Potomac River, and throughout this 
distance is one of the most important members of the Potomac 
group. It has a much larger areal extent than either of the 
two formations before described, and in places overlaps them, 
resting directly upon the crystalline rocks of the Piedmont 
Plateau. 
Leading features of the deposits—The deposits of the Patapsco 
formation consist chiefly of highly colored and variegated 
clays which grade over into lighter colored sands and clays, 
while sandy lenses of coarser materials are sometimes interstrati- 
fied, which are occasionally indurated and at times form “pipe 
