468 ORIGIN OF THE APPALACHIAN COAL STRATA, 



their eastern limit more than one hundred and fifty miles to the 

 west of the general eastern boundary of the upper ones ; and, as 

 we ascend in the formation, the beds extend successively more 

 and more to the east, or in the direction of the ancient land. But 

 considering the many striking instances which I have recorded, 

 of the close approach or actual contact of certain beds of coal and 

 oceanic limestone, we cannot resist the conclusion, that the grad- 

 ual downward movement was frequently interrupted by a slow 

 upward one. In all the instances that I have cited, where the 

 limestone sti'atum immediately underlies a coal-seam, it is obvious 

 that an upward movement of the land must have taken place, so 

 gradually as to be unattended by any sensible commotion of the 

 waters. A considerable and sudden lifting of the bed of the sea, 

 would infallibly have caused the production of violent currents, 

 competent to spread over the quiet precipitate of limestone, one 

 or more coarse arenaceous or argillo-arenaceous strata. That the 

 intervals of repose, indicated by the limestones, were, like those of 

 the beds of coal, sometimes suddenly terminated by earthquake 

 disturbances, strewing over the marine sediments the materials 

 of the land, is manifest from the phenomena, though it is not less 

 clear, that the cessation of the periods of relative tranquillity, marked 

 by very gradual subsidence, must, in all cases, where the coal- 

 beds are overlaid dii'ectly by marine limestones, have been effected 

 by simply a more rapid process of depression. Every superimpos- 

 ed limestone, without an intervening roof-slate, or sandstone, to 

 separate it from the coal, affords, I conceive, a conclusive proof of 

 this increased rate of submergence. Perhaps it will be objected, 

 that a merely accelerated subsidence, such as I have here suppos- 

 ed, if taken in conjunction with the hypothesis of a drifting of 

 the land materials by rivers, will satisfactorily explain all the facts 

 which I have ascribed to the turbulent movement of the sea against 

 the land during earthquakes. But though it is highly probable, 

 from the phenomena of nearly every extensive coal-field, that 

 rivers did carry into the parts of the ocean and its estuaries, now 

 drained and occupied by the coal strata, a considerable quantity 

 of argillaceous deltal deposits, yet it is difficult to imagine how 

 any moderately rapid subsidence, if unaccompanied by some 



