366 PALEOZOIC TIME — CARBONIFEROUS AGE. 



marine shells, — a fact observed in more than one case in the coal 

 of the United States. 



As already explained, there is no reason to suppose that the vege- 

 tation was confined to the lower lands : it probably spread over the 

 whole continent, to its most northern limits. It formed coal only 

 where there were marshes, or the deposits of vegetable debris be- 

 came covered by water-deposits of sand, clay, or other rock-material. 



4. Phases in the progressing Carboniferous period. — The con- 

 dition of the continent which has been described represents only 

 one phase in the Carboniferous period. The rocks register a suc- 

 cession of changes ; for coal beds are succeeded by sandstones, or 

 shales, or limestones, or iron-ore beds, and many alternations of these 

 beds, to a thickness fifty times as great as that of the coal beds. 

 These intervening strata, moreover, may be fresh-water or marine: 

 in the one case, with fresh-water shells or other inland species ; in 

 the other, full of Crinoids and Brachiopods, the life of the sea. The 

 great extent of the continent, wherever these strata occur, under- 

 went, therefore, continued oscillations of level, or the sea as un- 

 ceasing changes of water-level. After a period of verdure there 

 followed a desolation as complete as that when the lower Millstone 

 grit was spread over the surface, — either a subsidence of the interior, 

 or some other change that led to a general submergence beneath 

 fresh waters, or a movement or removal or sinking of barriers, 

 that placed the whole beneath salt water : in either case, the former 

 vegetation gave way to the water-life again, and the broken relics are 

 often packed together in the first deposits that ensued. The oscil- 

 lations must have been exceedingly various to have produced all 

 the alternations of shales, sandstones, limestones, and ore-beds. 

 They must have been also slow in progress: motion by the few 

 inches a century accords best with the' facts. The continent may 

 have rested long near the water's surface, just swept by the waves. 

 It may have been long a region of barren marshes ; and in this 

 condition it might have received its iron-ore deposits, as now marshes 

 become occupied by bog-ores. It must have been long in somewhat 

 deeper waters, and covered with a luxuriance of marine life. 

 Finally, the land escaped again from the waters, and the old vege- 

 tation spread rapidly across the great flats, commencing a new era 

 of coal-making vegetable debris ; or the escape was only partial, 

 and coal-plants took possession of one part and made limited coal 

 deposits, while the sea still held the rest beneath it ; for uniform 

 oscillations of level in all cases through so great an area are not 

 probable, and therefore the former continuity of a single coal bed 

 through the East and West requires strong proof to be admitted. 



