32 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



of plants, the position of these plants in the roof shale, the com- 

 pletely disorganized condition of materials in the coal, the presence 

 of the underclay, with roots and the occurrence of vertical stems 

 rooted in the underclay. The chief objection to the theory is the 

 repeated alternation, in the same locality, of coal seams with marine 

 and freshwater strata. There being as many as one hundred coal 

 seams, it would appear as though the same spot has been raised 

 above water level and had been depressed below it at least one 

 hundred times. 



The estuary theory was proposed to avoid this difficulty. As an 

 estuary at the mouth of a great river is occupied now by salt- and 

 again by fresh-water, it should contain alternating deposits of marine 

 and fresh-water origin. In seasons of freshet, the salt water is 

 pushed out and the river water, loaded with mineral detritus and 

 timber rafts, makes its deposits ; during low water, the sea returns 

 and marine deposits follow. 



Le Conte finds insuperable objections to the latter. He thinks 

 that coal beds were formed as peat bogs at the mouths of large 

 rivers. The analogy is to be sought, not in the bogs of Ireland, 

 but in those of the Mississippi delta. He supposes a vast delta, 

 with spaces protected by fringes of plants from influx of river muds. 

 There pure vegetable matter would accumulate until during some 

 violent flood the barrier would be broken down and the whole space 

 covered by mud. The delta, like that of the Mississippi, subsided 

 slowly and the covering of mineral detritus eventually became 

 ground for a new marsh. If the subsidence were more rapid than 

 the river deposits could overcome, the sea would take possession 

 and limestone would be formed. There is no necessity for con- 

 ceiving repeated upheavals and depressions. '* Coal has almost cer- 

 tainly accumulated in situ in extensive peat swamps at the mouths 

 of large rivers, upon ground which was slowly subsiding during the 

 whole period." 



Lesquereux,''^ after long study of peat bogs in Europe, came to 

 the United States, where as palaeobotanist to several official sur- 



'"' L. Lesquercux, PaLxontological report on fossil flora of the Coal 

 Measures, Third Ann. Rep. Geol. Survey of Kentucky. Frankfort, 1857, pp. 

 505-522. 



32 



