No similarly intercalated layer of marine sh.e.l.ls has been noticed in the neighbouring coal-field of Newcastle, where, as in South Wales and Somersetshire, the marine deposits are entirely below those containing terrestrial and freshwater remains.[326-A]

_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 const.i.tuting the shales. Mr. Hunt, of the Museum of Practical Geology, inst.i.tuted a series of experiments to ill.u.s.trate 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 carbonate 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.[326-B]

FOOTNOTES:

[308-A] Phillips; art. "Geology," Encyc. Britan.

[309-A] Sedgwick, Geol. Trans., Second Series, vol. iv.; and Phillips, Geol. of Yorksh. part 2.

[309-B] Memoirs of Geol. Survey, vol. i. p. 195.

[315-A] The trunk in this case is referred by Mr. Brown to _Lepidodendron_, but his ill.u.s.trations seem to show the usual markings a.s.sumed by _Sigillaria_ near its base.

[316-A] For terminology of cla.s.sification of plants, see above, note, p. 223.

[316-B] Quart. Geol. Journ., vol. v., Mem., p. 17.

[317-A] Anniv. Address to Geol. Soc., 1840.

[317-B] Hawkshaw, Geol. Soc. Proceedings, Nos. 64. and 69.

[318-A] Geol. Report on Cornwall, &c. p. 143.

[318-B] Lindley and Hutton, Foss. Flo. part 6. p. 150.

[319-A] See papers by Messrs. Beckett and Ick. Proceed. in Geol. Soc., vol. iv. p. 287.

[319-B] Annales des Mines, 1821.

[320-A] Principles of Geol., 8th ed., p. 215.

[321-A] See figures of texture, Witham, Foss. Veget., pl. 3.

[321-B] See Lyell's Travels in N. America, vol. ii. p. 179.

[322-A] Quart. Geol. Journ., vol. ii. p. 177.

[324-A] Geol. Quart. Journ., vol. ii. p. 393.; and vol. vi. p. 115.

[325-A] Prestwich, Geol. Trans., 2d Series, vol. v. p. 440. Murchison, Silurian System, p. 105.

[325-B] Silurian System, p. 84.

[325-C] Trans. Roy. Soc. Edin. vol. xiii. Horner, Edin. New Phil.

Journ., April, 1836.

[325-D] Phillips; art. "Geology," Encyc. Metrop., p. 590.

[326-A] Phillips; art. "Geology," Encyc. Metrop., p. 592.

[326-B] Memoirs of Geol. Survey, pp. 51. 255, &c.

CHAPTER XXV.

CARBONIFEROUS GROUP--_continued_.

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 Alleghanies--Mechanically formed rocks thinning out westward, and limestones thickening--Uniting of many coal-seams into one thick one--Horizontal coal at Brownsville, Pennsylvania--Vast extent and continuity of single seams of coal--Ancient river-channel in Forest of Dean coal-field--Absence of earthy matter in coal--Climate of carboniferous period--Insects in coal--Rarity of air-breathing animals--Great number of fossil fish--First discovery of the skeletons of fossil reptiles--Footprints of reptilians--Mountain limestone--Its corals and marine sh.e.l.ls.

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 their character in the low country to the westward of those mountains.

The annexed diagram (fig. 379.) will a.s.sist the reader in understanding the phenomena now alluded to, although I must guard him against supposing that it is a true section. A great number of details have of necessity been omitted, and the scale of heights and horizontal distances are unavoidably falsified.

[Ill.u.s.tration: Fig. 379. Diagram explanatory of the geological structure of a part of the United States between the Atlantic and the Mississippi.

Length from E. to W. 850 miles.

Appalachian Coal Field. Alleghanies, or Appalachians.

Same section--_continued_.

Mississippi. Illinois Coal Field. Cincinnati. Appalachian Coal Field.

A B. Atlantic plain.

B C. Atlantic slope.

C D. Alleghanies or Appalachian chain.

D E. Appalachian coal-field west of the mountains.

E F. Dome-shaped outcrop of strata on the Ohio, older than the coal.

F G. Illinois coal-field.

_h._ Falls and rapids of the rivers at the junction of the hypogene and newer formations.

_i_, _k_, _l_, _m_. Parallel folds of Appalachians becoming successively more open, and flatter in going from E. to W.

_References to the different Formations._

1. Miocene tertiary.

2. Eocene tertiary.

3. Cretaceous strata.

4. Red sandstone with ornithichnites (new red or trias?) usually much invaded by trap.

5. Coal-measures (bituminous coal).

5' Anthracitic coal-measures.

5'' Carboniferous limestone of the Illinois coal-field, wanting in the Appalachian.

6. Old red or Devonian, Olive slate, &c.

7. Primary fossiliferous or Silurian strata.

8. Hypogene strata, or gneiss, mica schist, &c., with granite veins.