Cordaites, of the Group of Dory-Cordaites. Branch Restored.—After Grand’ Eury.
CHAPTER IV.
the origin of plant life on the land.
f the graphite of the Laurentian rocks was derived from vegetable matter, the further question arises, Was this vegetation of the land, or of the sea? and something may be said on both sides of this question. If there were land plants in the Laurentian period, they must have grown either on rocks older than the Laurentian itself, or on such portions of the beds of the latter as had been raised out of the sea, forming perhaps swampy flats of newly-made soil. But we know no rocks older than the Laurentian, and there is no positive evidence that any of the beds of that formation were other than marine. Still it is not impossible that some of the beds which are now graphitic gneisses may originally have been similar to the bituminous shales, coals, or underclays of the coal formation. The graphite occurring in veins, if of vegetable origin, must have been derived from liquid bitumen oozing into fissures; and veins of this kind occur in later formations, both in marine and fresh-water beds. The only other positive argument which has been adduced in favour of the existence of abundant land plants in the Laurentian is that of Dr. Sterry Hunt, derived from the great beds of iron ore, which it is difficult to account for chemically except on the hypothesis of the decay in the air of great quantities of vegetable matter. The question must remain in doubt till some one is fortunate enough to find portions of the Laurentian carbon retaining traces of organic structure. My own observations, though somewhat numerous, allow me only to say that the graphite sometimes presents fibrous forms, that it occasionally appears as vermicular threads—which, however, I suppose to be fillings of canals of Eozoon—and that in the graphitic beds there are occasionally slender root-like bodies of a lighter colour than the mass; but none of these indications are sufficient to determine anything as to its vegetable origin, or the nature of the plants from which it may have been derived.
In any case, the quantity of carbon which has been accumulated in the Laurentian rocks is very great. I have measured one bed at Buckingham, on the Ottawa, estimated to contain 20 per cent. of carbon, and which is at least eight feet in thickness. Sir William Logan has described another similar bed from ten to twelve feet thick, and more recent reports of the Geological Survey of Canada mention a bed supposed to be twenty-five feet thick, in which Mr. Hoffman finds 30 per cent. of carbon. On the whole the quantity of carbon in the graphitic zone of the Laurentian is comparable with that in certain productive coal-fields, and we certainly have in the subsequent geological history no examples of such accumulations except from remains of the luxuriant vegetation of swampy flats.