Ch. xxy.] coal-fields of united states. 387 



Somersetshire, the marine deposits are entirely below those containing 

 terrestrial and freshwater remains.* 



Clay -iron-stone. — Bands and nodules of clay-iron-stone are common 

 in coal-measures, and are formed, says Sir H. De la Beche, of carbonate 

 of iron, mingled mechanically with earthy matter, like that constituting 

 the shales. Mr. Hunt, of the Museum of Practical Geology, instituted 

 a series of experiments to illustrate the production of this substance, and 

 found that decomposing vegetable matter, such as would be distributed 

 through all coal strata, prevented the farther oxidation of the proto-salts 

 of iron, and converted the peroxide into protoxide by taking a portion 

 of its oxygen to form carbonic acid. Such carbonic acid, meeting with 

 the protoxide of iron in solution, would unite with it and form a car- 

 bonate of iron ; and this mingling with fine mud, when the excess of 

 carbonic acid was removed, might form beds or nodules of argillaceous 

 iron-stone.f 



CHAPTER XXV. 



CARBONIFEROUS GROUP COntltlUed. 



Coal-fields of the United States — Section of the country between the Atlantic 

 and Mississippi — Position of land in the carboniferous period eastward of the 

 Alleghanics — Meclianically formed rocks thinning out westward, and limestones 

 thickening — Uniting of many coal-seams into one thick bed — Horizontal coal 

 at Brownsville, Pennsylvania- — Vast extent and continuity of single seams of 

 coal-T-Ancient river-channel in Forest of Dean coal-field — CHmate of carbo- 

 Eifcrous period — Insects in coal — Rarity of air-breathing animals — Great num- 

 ber of fossil fish — First discovery of the skeletons of fossil reptiles — Footprints 

 of reptilians — First land-shell found — Rarity of air-breathers, whether verte- 

 brate or invertebrate, in Coal-measures — Mountain limestone — Its corals and 

 marine shells. 



It was stated in the last chapter that a great uniformity prevails in 

 the fossil plants of the coal-measures of Europe and North America ; 

 and I may add that four-fifths of those collected in Nova Scotia have 

 been identified with European species. Hence the former existence at 

 the remote period under consideration (the carboniferous) of a continent 

 or chain of islands where the Atlantic now rolls its waves seems a fair 

 inference. Nor are there wanting other and independent proofs of such. 

 an ancient land situated to the eastward of the present Atlantic coast of 

 North America ; for the geologist deduces the same conclusion from the 

 mineral composition of the carboniferous and some older groups of rocks 

 as they are developed on the eastern flanks of the Alleghanies, contrasted 

 with|,tlieir character in the low country to the westward of those moun 

 tains. 



The annexed diagram (fig. 505) will assist the reader in under- 



* Phillips ; art. " Geology," Encyc. Metrop. p. 592. 

 f Memoirs of Geol. Survey, pp. 51, 25o, &c. 



