carbonaceous débris, which seems to be the remains of either aquatic or land plants, is locally not infrequent.
Referring to the land vegetation of the older rocks, it is difficult to picture its nature and appearance. We may imagine the shallow waters filled with aquatic or amphibious rhizocarpean plants, vast meadows or brakes of the delicate Psilophyton and the starry Protannularia and some tall trees, perhaps looking like gigantic club-mosses, or possibly with broad, flabby leaves, mostly cellular in texture, and resembling algæ transferred to the air. Imagination can, however, scarcely realize this strange and grotesque vegetation, which, though possibly copious and luxuriant, must have been simple and monotonous in aspect, and, though it must have produced spores and seeds and even fruits, these were probably all of the types seen in the modem acrogens and gymnosperms.
"In garments green, indistinct in the twilight,
They stand like Druids of old, with voices sad and prophetic."
Prophetic they truly were of the more varied forests of succeeding times, and they may also help us to realize the aspect of that still older vegetation, which is fossilized in the Laurentian graphite; though it is not impossible that this last may have been of higher and more varied types, and that the Cambrian and Silurian may have been times of depression in the vegetable world, as they certainly were in the submergence of much of the land.
These primeval woods served at least to clothe the nakedness of the new-born land, and they may have sheltered and nourished forms of land-life still unknown to us, as we find as yet only a few insects and scorpions in the Silurian. They possibly also served to abstract from the atmosphere some portion of its superabundant carbonic acid harmful to animal life, and they stored up supplies of graphite, of petroleum, and of illuminating gas, useful to man at the present day. We may write of them and draw their forms with the carbon which they themselves supplied.