. Elements of geology : a text-book for colleges and for the general reader. Geology. THEORY OF THE ACCUMULATION OF COAL. 375. Fig. 516. -Ground-Plan of a Fossil Forest. Parkfield Colliery. ing drift-timber), and limestones, interstratified with the coal. The phenomena of an individual seam prove the accumulation by growth in situ; the general phe- nomena of a coal-basin, with its succession of strata, prove that this took place at the mouths of rivers. Thus, the field of discussion is nar- rowed to very small limits. We conclude, therefore, that coal has been accumu- lated in extensive peat-
. Elements of geology : a text-book for colleges and for the general reader. Geology. THEORY OF THE ACCUMULATION OF COAL. 375. Fig. 516. -Ground-Plan of a Fossil Forest. Parkfield Colliery. ing drift-timber), and limestones, interstratified with the coal. The phenomena of an individual seam prove the accumulation by growth in situ; the general phe- nomena of a coal-basin, with its succession of strata, prove that this took place at the mouths of rivers. Thus, the field of discussion is nar- rowed to very small limits. We conclude, therefore, that coal has been accumu- lated in extensive peat- swamps at the mouths of great rivers, and therefore subject to occasional flood- ings by the river and inun- dations by the sea. That pure peat may accumulate under these cir- cumstances, is sufficiently proved by the fact mentioned by Lyell, that over large tracts of ground in the river-swamp and delta of the Missis- sippi pure peat is now forming, in spite of the annual floods; the sedi- ments being all stopped by the thick jungle-growth surrounding these spots, and deposited on the margins, while only pure water reaches the interior portions.* But if coal has indeed been formed at the" mouths of great rivers, we ought to find at least something analogous to a coal-field in sections of great ri^er-deltas. And so, indeed, we do. We have seen (p. 136) that a great river-delta, like that of the Mississippi or the Ganges, consists of alternate layers of river-sediments (sands and clays) and marine sedi- ments (limestones) with thin layers of peaty matter, and old forest- grounds with stumps and roots. It is, in other words, a coal-field, though an imperfect one, in the process of formation. It will be re- membered, also, that we accounted for this alternation, not by oscilla- tions, but by the operation of two opposing forces, one depressing (sub- sidence), the other up-building (river-deposit), with varying success. When the up-building by river-deposit prevailed, the area
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