456 
ELEMENTS OF GEOLOGY. 
carbonaceous shale, about three inches in thickness. It rests 
on an under-clay in which are the roots of Psilophyton (see 
Fig. 523). At many other levels rootlets of this same plant 
have been shown by Principal Dawson to penetrate the clays, 
and to play the same part as do the rootlets of Stigmaria in 
the coal formation. 
We had already learnt from the works of Goppert, Unger, 
and Bronn that the European plants of the Devonian epoch 
resemble generically, with few exceptions, those already 
known as Carboniferous; and Dr. Dawson, in 1859, enumer¬ 
ated 32 genera and 69 species which he had then obtained 
from the State of E’ew York and Canada. A perusal of his 
catalogue,* comprising Coniferce^ Sigillarice^ Catamites^ Aste- 
rophyllites^ Lepidodendra^ and ferns of the genera Cydopteris^ 
Neuropteris^ Splienopteris^ and others, together with fruits, 
such as Cardiocarpiim and Tngonocarpum^ might dispose 
geologists to believe that they were presented with a list of 
Carboniferous fossils, the difference of the species from those 
of the coal-measures, and even a slight admixture of genera 
unknown in Europe, l3eing naturally ascribed to geographical 
distribution and the distance of the New from the Old World. 
But fortunately the coal formation is fully developed on the 
other side of the Atlantic, and is singularly like that of Eu¬ 
rope, both lithologically and in the species of its fossil plants. 
There is also the most unequivocal evidence of relative age 
afforded by superposition, for the Devonian strata in the 
United States are seen to crop out fi'om beneath the carbon¬ 
iferous on the borders of Pennsylvania and New York, where 
both formations are of great thickness. 
The number of American Devonian plants has now been 
raised by Dr. Dawson to 120, to which we may add about 80 
from the European flora of the same age, so that already the 
vegetation of this period is beginning to be nearly half as rich 
as that of the coal-measures which have been studied for so 
much longer a time and over so much wider an area. The 
Psilophyton above alluded to is believed by Dr. Dawson to 
be a lycopodiaceous plant, branching dichotomously (see P, 
princeps^ Fig. 523), with stems springing from a rhizome, 
which last has circular areoles, much resembling those of 
Stigmaria, and like it sending forth cylindrical rootlets. The 
extreme points of some of the branchlets are rolled up so as 
to resemble the croziers or circinate vernation of ferns; the 
leaves or bracts, a, supposed to belong to the same plant, are 
described by Dawson as having inclosed the fructification. 
The remains of Psilophyton princeps have been traced through 
* Quart. Geol. Journal, vol. xv., p. 477,1859 ; also vol. xviii., p. 296,1862. 
