506 CLARK AND BIBBINS 
sequently placed all of the Coastal Plain deposits beneath the 
marine Cretaceous in the same geologic division.* Professor 
Marsh claims the more recent discovery of a large amount of 
vertebrate material which throws much light upon the age of 
the Potomac formation and which he proposes shortly to describe. 
The last contribution of Professor Marsh brought about an 
extended discussion regarding the age of the Potomac group 
which was participated in by Messrs. Gilbert,? Marcou,3 Hollick, 
Hill,4 and Ward.5 
The important postumous work of Professor J. S. Newberry 
entitled “The Flora of the Amboy Clays’’® is a valuable con- 
tribution to the geology of the New Jersey portion of the Raritan 
formation. This work, which was edited by Arthur Hollick, 
represents the results of many years of investigation on the part 
of the late Professor Newberry. 
The conclusions of the authors of this paper, as set forth in 
the preceding pages, are formed upon an extensive study of 
the Potomac group both in Maryland and New Jersey, and are 
seen to be quite different from those advanced by their predeces- 
sors. The comparative table on page 505 exhibits in a graphic 
manner the taxonomy of the several writers referred to, as inter- 
preted by the authors of this paper. 
Wm. BULLOCK CLarRK, 
GEOLOGICAL LABORATORY, ARTHUR BIBBINS. 
JoHNs HOPKINS UNIVERSITY, 
July 1897. 
tJurassic Formation of the Atlantic Coast, Amer. Jour. Sci., 4th ser., Vol. II, 
1896, pp. 433-447; Science, n. ser., Vol. IV, pp. 805-816. 
2 Science, n. ser., Vol. IV, 1896, pp. 875-877. 
3 Jbid., pp. 945-947; Vol. V, n. ser., 1897, pp. 149-152. 
4 Jbid., pp. 918-922. 
5 [bid, 1897, pp. 411-423. 
© Monographs U. S. Geol. Surv., Vol. XX VI, Washington, 1895. 
