100 
Fossil Wood and Lignites — Knowlton. 
evidence is at hand. It was called by Rogers the Jurasso-Cre- 
taceous or Upper Secondary sandstone. In 1885 Mr. W, J. 
McGee, arguing mainly from the then available paleobotani- 
cal evidence, considered it to be “Lower-Cretaceous in age—the 
American equivalent of the European Neocomian.” Prof, Wm_ 
M. Fontaine of the University of Virginia, who has so 
thoroughly worked up the plant impressions, regards it as 
Wealden, while Prof. 0. C. Marsh who has studied the numer¬ 
ous vertebrate remains, claims for it a Jurassic age. 
It is remarkable for containing the oldest dicotyledonous- 
flora yet discovered. Of the 365 species of plants described 
from the Potomac formation by Prof. Fontaine, no less than 75' 
species are dicotyledons. They do not consist of the highly 
differentiated genera and species which characterize the other- 
dicotyledonous floras, such as the Dakota group, but are new 
and archaic in appearance, showing that this class had, as has 
been argued by Prof. Ward, an ulterior period of development 
and transition. 
My own studies of the Potomac flora have been exclusively 
devoted to an investigation of the internal structure of the* 
lignite and silicified wood which is very abundant in this forma¬ 
tion, and in this connection it may be well to speak of their 
mode of occurrence. 
The Potomac formation, which has an aggregate thickness 
probably of more than 400 feet, is readily divisible into two** 
members, an upper, called by Prof. Fontaine the clay-member r 
and a lower called the sandstone-member. The upper member 
contains little plant life and the material upon which the 
following observations are made, as also those by Fontaine, came 
wholly from the lower member. 
The remains occur principally in lenticular pockets of hard,- 
bluish clay, which pockets bear evidence of having been trans¬ 
ported en masse from the original beds in which they were 
laid down. These pockets \ary in their dimensions, some being 
only a few feet in length and one or two feet in thickness r 
while others are from ten to fifteen yards long and from three 
to ten feet thick. It is more than probable that originally this 
material was deposited in shallow water, which was-fresh, or at 
most but slightly brackish. An unknown thickness, filled 
with the debris of vegetable growth, was here accumulated,. 
