482 THE PRIMITIVE VEGETATION OF THE EARTH. 
organization. Until confirmed by other facts this discovery 
may be received with doubt, but I believe it can be relied 
on. 
Our knowledge of the flora of the Upper Silurian is at 
present nearly in the same state with that of the Middle and 
Lower Devonian ten years ago. I know in the Upper Silu- 
rian of Canada but two species of Psilophyton, both appar- 
ently identical with Devonian forms. In England, besides 
the spore-cases known by the generic name Pachytheca, 
there exists in the collections of the Geological Survey frag- 
ments of wood and bark which I believe indicate two 
additional species. In Germany three or four species are 
known in rocks of this age. All of these plants appear to 
be Acrogens allied to Lycopodiaceze. That these few spe- 
cies constitute the whole flora of the Upper Silurian we can 
scarcely believe. They occur in marine formations, and 
were probably drifted far from the somewhat limited land- 
surfaces which existed in the explored parts of the Upper 
Silurian areas. When we obtain access to deposits of this 
age formed in shallows or estuaries, we may hope to find a 
flora of greater richness; and, judging from present indica- 
tions, not dissimilar from that of the Lower Devonian. 
With the exception of some remains which I believe to be 
of very doubtful character, the Lower Silurian, has as yet 
afforded no remains of land plants, and in North America, 
at least, this is very significant, inasmuch as we have, in the 
Potsdam sandstone, extensive sandy flats of this period, in 
which we might expect to find drifted trunks of trees, if 
such had existed. But the search is not hopeless, and we 
may yet find some estuary deposit on the margin of the an- 
cient Laurentian continent, in whose beds the plants of that 
old land may occur. 
Lastly, for reasons stated in a paper lately published in 
the Proceedings of the Geological Society, I believe that the 
extensive deposits of graphite, which exist in the Laurentian 
of Canada, are of vegetable origin, and possibly in part 
