590 TIMOTHY WILLIAM STANTON 
sented by a Potomac species. According to Lydekker Pleuro- 
coelus also occurs in the Wealden of England. In recent papers 
Professor Marsh’ has attempted to establish the Jurassic age 
of the whole Potomac formation in its broadest sense. He 
correlates it by the relationship of the vertebrate faunas on the 
one hand with the Atlantosaurus beds, and on the other with the 
European Wealden, asserting that they are homotaxial equiva- 
lents. Assuming that the Atlantosaurus beds are Jurassic, it 
consequently follows, according to his reasoning, that the 
Wealden and the Potomac are also Jurassic. But even admit- 
ting that the vertebrate faunas are so closely related as to estab- 
lish the equivalence of the deposits in these three widely 
separated regions, the correlation can apply only to the beds in 
which the fossils occur, and in the Potomac they have so far 
been found only in the lower portion. Recent stratigraphic 
studies by Professor W. B. Clark and Mr. Arthur Bibbins on the 
Potomac in Maryland, have an important bearing on this ques- 
tion. A preliminary statement of their results, including a new 
classification of the deposits, was published? some months ago, 
and a more detailed account has been issued since the present 
paper was first written. Their most important results are sum- 
marized as follows: 
“Tt is the conclusion of the authors, founded upon a detailed 
stratigraphic study of the Potomac group, that all the beds 
which have afforded dicotyledonous types of plant life are 
above those from which Professor Marsh has obtained vertebrate 
remains, and moreover, that a marked unconformity exists 
between the two series of deposits. .... 
“The several formations into which the larger unit of the 
Potomac group has been divided, are as follows: 
tAm. Jour. Sci., 4th ser., Vol. II, 1896, pp. 295-298, 375-377, 433-447. See also 
articles on the same subject by HOLLICK, WARD, HILL, and GILBERT, Science, Vols. 
IV and V, 1896, 1897. 
? Physical Features of Maryland; Maryland Geol. Surv., Vol. I, Pt. 3, pp. 56-59, 
April 1897. 
3JouR. GEOL., Vol. V, pp. 479-506, July-Aug. 1897. 
