488 FOSSIL PLANTS FROM MONTANA—FONTAINE. 
based on the presumed absence of groups of plants, can not be fairly 
drawn until it is made certain that no further discoveries can be 
looked for. 
With regard to the age and affinities of this flora, my investigations 
add nothing to the conclusions arrived at by Dr. Newberry in the paper 
before cited. These conclusions, which I fully indorse, are that the 
Potomac group, the Great Falls group, the Kootanie group of Can- 
ada, and the Kome group of Greenland are all of the same general 
age. Dr. Newberry expresses the opinion that the Great Falls strata 
are somewhat older than those of the Potomac, this being indicated by 
the absence of angiosperms in the former. This opinion is justified if 
we take into consideration only the plants found in the Great Falls 
strata up to this time. But it is possible that, if these beds have not 
been exhaustively explored, angiosperms may yet be found in them. 
The strata seem to some extent to show an isolation of forms, and a 
confinement of them to particular horizons. This would seem to be the 
case with the cyeads. As stated before they do exist in the strata at 
a particular horizon, or at a particular locality, while in the two col- 
lections before me not a single imprint is found. In my collecting from 
the Potomac beds, I did not discover the angiosperms of modern type 
that they yield until towards the close of my explorations, long after 
large collections of plants of older types had been made. The more 
modern angiosperms of the Potomac are found in the upper beds of the 
formation, which have been in most places carried away by erosion 
There are localities in the Potomac of Virginia which show good expo 
sures of thick beds, that abound in impressions of ferns and conifers, 
but contain no angiosperms. The two near Potomac Run described 
in Monograph xv of the U.S. Geological Survey; as “Roadside” and 
‘¢ Hillside,” have this character. It is noteworthy that Osmunda dick- 
sonioides, a plant that. seems to be abundant at a certain horizon in the 
Great Falls field, is found at one of these localities and nowhere else 
in the Potomac beds. If the ageof the Potomac strata had been deter- 
mined from the fossils of these localities, it would, from the absence 
of angiosperms, appear older than it is. 
With regard to the age of the Potomac itself perhaps a word may be 
allowed here. It is to be understood that by Potomac is meant the 
lower member of that formation, as shown chiefly in Virginia. I do 
not regard this member as all of Wealden age. The period of its depo- 
sition seems, if we are to judge by the progress made by the plants, to 
have extended through the Wealden into the Urgonian, and perhaps to 
a somewhat later time. It was probably extended throughout the 
Neocomian. The very remarkable isolation and grouping of the plants 
of the Virginia Potomac, which seems in a measure to exist in the Great 
Falls field, and the great differences in the relative abundance of the 
different types, appear to indicate unstable conditions in the different 
elements of the flora, and also rapid changes. It is quite possible, then, 
