Fossil Wood and Lignites — Knowlton. 99 
the Carboniferous genus Phillipsia, permits one to follow the 
evolution very easily. In the upper layers Phillipsia mera- 
mecensis is associated with numerous other Keokuk forms, 
especially bryozoa. 
In like manner a succession of Lingulae from forms like L. 
subspatulata to a close analogue of L. scotia of the Coal 
Measures affords the same kind of evidence. The lamelli- 
branchs begin with Pterinopecten and Lyriopecten of Devonian 
habitus and by simple and easily-followed graduations lead to 
species of Aviculopecten and Crenipecten, while Leiopteria gives 
place to species of Avicula or the like. 
The evidence of other groups is the same. Indeed, indi¬ 
vidually we rise from the study with the belief in the continuity 
of lines of evolution strongly fortified and the last vestige of 
leaning towards the geological dogma of cataclysms removed 
forever. 
The plates accompanying are intended to convey an idea of 
the variety and character of the fauna as well as to incidentally 
illustrate these brief remarks. 
Denison University , Granville , Ohio. 
THE FOSSIL WOOD AND LIGNITES OF THE 
POTOMAC FORMATION.* 
By F. II. Knowlton. 
Perhaps no American geological formation, which has been 
made the subject of recent investigation, has given rise to more 
extensive discussion or has furnished more valuable scientific 
results, than has the Potomac formation. First clearly differ¬ 
entiated by Prof. W. B. Rogers as long ago as 1840, it has 
during the past decade, and more particularly during the last 
three years, been made a special study by Messrs McGee, Fon¬ 
taine, Ward, and Marsh, and at the present time the history of 
its deposition and abundant animal and plant life, is better 
known than is the history of many of the European formations 
with which it has usually been correlated. Its stratigraphic 
position however is still unsettled although strong presumptive 
*Paper read before Am. Asso. Advance Sci. Cleveland meeting, Aug,, 
1888. Resume of Bulletin U. S. Geol. Survey, (in press). 
