504 CLARK AND BIBBINS 
nate Clay Sands” placed at the top of the group as part of the 
Albirupean. 
The senior author of this paper, in a study of the Coastal 
Plain formations of New Jersey, proposed in 1892 the name 
“Raritan formation’’* for the ‘Plastic Clays” of Professor 
Cook, the term Raritan having been somewhat loosely used by 
several authors in earlier years in speaking of the clay deposits 
of this formation. 
Mr. N. H. Darton,? of the United States Geological Survey, 
as a result of his study of the Coastal Plain in southern Mary- 
land, differentiated a part of the sands at the top of the Potomac 
group as the ‘“‘Magothy formation,” which he considered might 
be equivalent to Professor Uhler’s ‘Alternate Clay Sands.” 
The elaborate investigations of Professor Ward, which began 
as already described in 1885, have been continued to the present 
day. His exhaustive researches upon the fossil plants in the 
Potomac group have added largely to our knowledge of that 
formation. His most important publication? appeared in 1895, 
in which the Potomac formation is subdivided into a number of 
series, ViZ.:— 
Albirupean Series, Newer Potomac. 
Iron Ore aS 
Aquia Creek ‘“ 
Mt. Vernon ‘“ Older Potomac. 
Rappahannock Series, 
James River is 
__~,-_ 
The interest aroused in the age of the Potomac formation 
led tothe collection by Mr. J. B. Hatcher under the direction of 
Professor O. C. Marsh‘ of the United States Geological Survey 
of vertebrate remains from the iron ore beds in the vicinity of 
Muirkirk, Prince George’s county, Maryland. Upon the basis 
of these remains Professor Marsh has unequivocally referred 
the Potomac group of Maryland to the Jurassic. He has sub- 
t Ann. Rept. State Geol., N. J., 1892 (1893), pp. 181-186. 
2 Amer. Jour. Sci., 3d ser., Vol. XLV, 1893, pp. 407-419. 
315th Ann. Rept. Dir. U.S. Geol. Survey 1893-4 (1895), pp. 307-397. 
4 Amer. Jour. Sci., 3d ser., Vol. XX XI, 1888, pp. 89-97. 
