we know not if the proportion of carbonic acid be now constant in the air, and must admit that a reconversion of all the coal to carbonic acid gas would give a very large addition of this gas to the atmosphere,) we shall understand how the vegetation of the carboniferous period might be even more abundant than that now seen between the tropics, and at the same time comprehend the possibility of few land animals existing on the globe. Within what limits of proportion of carbonic acid in the air plants and animals can live, we do not know; but in this respect they are reciprocally circumstanced, —plants require most, animals require least.
De Luc, Brongniart, and other writers, prefer to explain the origin of coal from somewhat like peat-bogs, or from the decay or overwhelming of forests in sitû: if we admit further so much of water drift as the case requires, we have a general explanation. In most coal districts are from 20 to 60 seams of coal, alternating with sandy and argillaceous strata; each of these coal seams is composed of many parallel layers of different quality and structure, often separated by scattered patches and fragments of woody fibre. A bed or seam of coal is, in fact, an aggregate of many successive deposits of vegetable matter. Under almost every bed of coal, as Mr. Logan in particular has shown in South Wales, is a peculiar fire-clay, or a fine-grained sandstone (ganister in Yorkshire) traversed by the rootlets of a plant (stigmaria): over some beds of coal stand erect the stems of sigillaria and lepidodendron; and finally, at Dixonfold, near Manchester, and in other places, lepidodendra and sigillariæ rise erect out of a bed of coal, the former being connected below with roots which are stigmaria. In such cases, we seem to behold a nearly level swamp, often many miles in breadth, and of great longitudinal extent, covered with a peculiar deposit from water (the under clay), on which grew lepidodendra and other trees of the period. Spreading their symmetrical roots and rootlets through this mud, and rising to a considerable height above, a peaty accumulation happened around them,
