480 CLARK AND BIBBINS 
sequence of the Potomac deposits, together with the horizons 
from which the fossils have been derived, if the questions at 
issue are to be finally settled. The authors of this paper have 
been engaged upon the investigation of these relations during 
the past year, and believe that much of the difference of opinion 
is due to the lack of knowledge regarding the stratigraphic 
position of the beds which have yielded the various plant and 
animal remains. They desire at the outset, however, to express 
their obligations to their predecessors in the field, without the 
results of whose work their own investigations would have been 
seriously retarded, if not rendered entirely abortive. The great 
volume of data which the paleobotanists have presented to us 
during the past few years, and the more meager evidence of the 
vertebrate paleontologists, have been of signal service in inter- 
preting the stratigraphy of the Potomac formation. It is a 
pleasure to witness to the splendid achievements of Professors 
Ward, Fontaine, and Newberry, in the study of the fossil floras, 
and of Professor Marsh in extricating from poorly fossiliferous 
beds the important vertebrate remains which he states he has in 
store for us. The junior author of this paper has also made 
collections of the flora and fauna which will be discussed by him 
in a subsequent contribution. 
The conclusions reached by those who have studied these 
two classes of organic remains may be briefly stated as follows: 
The palzobotanists, largely upon the discovery of dicotyledon- 
ous types of plant life, claim the Cretaceous age of the Potomac 
group, while Professor Marsh upon the evidence of the verte- 
brate remains, particularly of the Dinosauria, is as firmly con- 
vinced of the Jurassic age of the deposits. 
It seems to the authors that the difficulty lies in the fact that 
each side has assumed too largely the unity of the Potomac 
group and has not sufficiently regarded the possibility of its rep- 
resenting more than a single formation. A marked exception 
to this is found in the late work of Professor Ward who has 
discovered several distinct stages in the fossil floras—a dis- 
crimination which is of much importance in determining the 
