For years the subject of the origin of coal formed one of the many battle-fields on which geologists delighted to break lances. They ranged themselves under two banners, the "drift"-theory men and the "growth"-theory men, the former maintaining strenuously that coal was simply vegetation transported from the land and deposited in large troughs at river-mouths or sea-bottoms, the latter as eagerly contending that the vegetation had not been drifted, but grew on the very locality where its remains are now exhumed. Neither party lacked plausible arguments in support of its doctrines. The "drift" combatants stoutly affirmed it to be contrary to all experience that a land-surface should be so oscillating as their opponents required, that in short it was absurd to hold each coal-seam as marking a period of elevation, for there were often dozens of seams in as many yards of strata, some of them scarcely an inch thick, and yet, according to the "growth" theory, each would have required for its accumulation a special uplifting of the land above the sea-level. These and many other difficulties were thought to be triumphantly overcome by the hypothesis of transport and deposition. The vegetation borne down by some ancient Mississippi would collect in vast rafts, and these becoming water-logged would sink to the bottom, where, by getting eventually covered over with silt and sand, they would in process of time be chemically altered into coal. This explanation was, however, vigorously resisted by the opposite side. They alleged that the "drift" theory could account neither for the wide extent of coal-seams nor for their remarkable persistency in thickness. If the vegetation had really been hurried out to sea by river-action, it seemed natural to expect that the coal-seams should occur in sporadic patches of very unequal thicknesses, according as the drifted plants had been more densely or more loosely packed. But this was found not to be the case in point of fact. The coal-seams were ascertained to be generally singularly continuous, and to retain for the most part a pretty uniform thickness over considerable areas. And what was still more worthy of note, they were, as a whole, markedly free from extraneous matter, such as sand and mud. Where these impurities did occur, it was usually in the form of intercalated seams or partings, often quite as regular and extensive as the coal itself. Had the vegetation, therefore, been transported into the sea, it could hardly fail to get mixed up with the fine impalpable mud which, like that of the Ganges or Mississippi, might have discoloured the ocean for leagues from the river-mouth, and settled down as a thickening stratum at the sea-bottom. And many other arguments, derived from the nature and arrangement of the strata interbedded among the coal-seams, were urged to prove that the latter had originated from vegetation which grew on the spot.