PRE-CARBONIFERODS PLANTS. 



67 



glahrum of the Devonian beds ; but they are too imperfect to show their 

 generic affinities. (Fig. 2^9). Others are branching stems, the form and 

 markings of which show that they belong to Psilophi/ton, and probably to 

 the species P. robustiua, (Fig. 243) . Others are obviously Rhizomata of 

 Psilophyton, sb owing the ramenta and areoles ; and one specimen collected 

 by Mr. R. Bfll, when exploring these rocks in connection with the 

 Geological Survey, shows the scalariform axis, and outer fibrous bark 

 in excellent preservation, thus fully proving its true nature, and vindicating 

 the less perfect specimens from the doubts which in the minds of many 

 botanists might otherwise rest upon them. This specimen is represented 

 in PI. XX, Figs. 241, 242 ; and I have shown its structure in PI. XI, 

 Figs. 133 and 134. 



These remains of Psilophi/ton occur in the lower part of the limestone, 

 but are more abundant in the upper beds, and they suffice to indicate the 

 existence of neighbouring land, probably composed of the Lower Silurian 

 rocks, and supporting vegetation. That this vegetation consisted wholly 

 of plants of the genus Piilophyton we are under no necessity to believe, 

 as the almost exclusive prevalence of these plants in some parts of the 

 overlying sandstones, deposited at a time when we know that other plants 

 existed, shows that in the Devonian period itself, this particular genus wa« 

 either much more abundant than other forms of plants, or much more 

 favourably situated for preservation. 



It is interesting to observe that just as a few remains of plants in the 

 marine limestones testify to the occurrence of neighbouring land, so an 

 occasional shell of Orthoceras or a few Brachiopods may sometimes be 

 found in the plant-bearing Devonian sandstones, showing that these were 

 accessible to the driftage of oceanic remains ; but we have no right in 

 either case to assume that these exceptional remains represent the wealth 

 of either the land or the sea in organic forms. 



III. General Remarks and Conclusions. 

 (1.) Physical Condition of Eastern America in the Devonian Period. 



As Hall and Dana have pointed out,* North America presented, in 

 the earlier part of the Upper Silurian period, a great internal ocean, 

 partially separated from the basin of the Atlantic by a more or less contin- 

 uous belt of sand-banks or islands, representing the older portions of the 

 Appalachian range of hills, and bounded on the north by Laurentian and 

 in part by Lower Silurian land. These conditions prevailed during the 

 deposition of the great Niagara limestones (Wenlock) ; and in the imme- 



• Report on Pal. of N. York, Vol. III.— Manual of Geology. 



