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 was reclaimed, 

 and became covered with thick jungle vegetation ; when the subsidence 

 prevailed, it was again covered with water, and buried in river-sedi- 

 ments, etc. Now and then, when the subsidence was unusually great, 

 the sea invaded the same area, and limestone was formed. It is sub- 

 stantially in this way that coal-fields were probably formed. 



Application of the Theory to the American Coal-Fields : a. Appala- 

 chian Coal-Field. — A glance at the map (p. 291) will show that, during 



* Lyell, Elements of Geology, p. 488. 



