BITUMINOUS AND ANTHRACITIC. 467 



as the limestones, cherts, and beds of carbonate of iron. The 

 analysis already given of our Appalachian coal-measures, will be 

 seen to imply a slow general subsidence, alternating with occa- 

 sional and less prolonged movements of elevation ; these gentle 

 changes of level, interrupted by sudden or paroxysmal heavings 

 of the crust. Mr. Beaumont was the first, I believe, to su safest a 

 subsidence of the land during the progress of the coal-formation ; 

 he supposes the coal-beds to be the result of a " luxuriant vegeta- 

 tion, covering swampy islands, which, by the settling down of the 

 disturbed crust of the earth, were covered over with drifted sand, 

 clay, &c." Subsequently Mr. Lyell, in the last edition of his 

 Elements of Geology, proposes a somewhat similar view. He 

 says, " If the superposition on a great scale of purely marine strata 

 to others containing coal and fresh-water shells, leads us to infer 

 that large areas, once constituting estuaries, deltas, and marshes, 

 sank down and became sea during the carboniferous period, so 

 are there reasons for concluding, that in many cases the depres- 

 sion of the ground took place gradually, and that in consequence 

 of the deposition of sediment, the same space was again and 

 again converted into land and laid under water." In another 

 passage he suggests, that " If we appreciate the full strength of 

 the evidence in favor of continued subsidence in the coal-field of 

 South Wales, we shall be the less surprised to learn that the ver- 

 tical depth of the superimposed sh'ata is enormous, amounting in 

 some places to no less than twelve thousand feet."* Though a 

 vast preponderance of subsidence over elevation is plainly indi- 

 cated in the prodigious thickness of the coal-measures, each par- 

 ticular coal-seam in which was produced successively at the sur- 

 face, I cannot conceive that either an alternation of periods of 

 subsidence and repose, or an uninterrupted prolonged depres- 

 sion, will explain the phenomena of the Appalachian coal rocks, 

 as they have been here described. A general subsidence through- 

 out the coal period, of all the great area now occupied by the 

 Appalachian basins, is proved independently of the above evidence 

 derived from the nature of the coal-beds, by the interesting fact, 

 that the lower seams of Ohio and western Pennsylvania, have 



* Lyell's Elements, Bost. edit., Vol. II., pp. 128 and 134. 



