198 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



originated on the land side and in fresh water. The occurrence of marine mollusks 

 in the roofs of coal beds is far more common than in the partings of the coal. 

 The generally local occurrence and variable character of the partings may be 

 construed as showing that the inroads of the sea were restricted. In this connec- 

 tion it may be noted that the No. 6 coal of Illinois, which extends with unusual 

 regularity of thickness and bedding structure over several counties in the southern 

 part of the State, carries a remarkable persistent argillaceous or slightly sandy 

 and often 'pyritic' parting known as the 'blue band,' about 0.75 inch to 3 inches 

 thick, over nearly the whole area. 



"The usually rather high ash-content of the coal of the eastern Interior basin, 

 the generally high percentage of sulphur, and the deposition of silica in the 

 organic mass, possibly as the result of precipitation of colloids by the overflow of 

 alkaline waters, appear to the writer to be more or less directly associated with the 

 conception of relative freedom of access of sea or brackish waters, and thus to 

 accord with the occurrence and character of the black carbonaceous roof muds 

 with their marine invertebrate contents which mark the permanent inundation 

 of the peat deposit by the sea. It is not impossible that in these regions of 

 most intimate relation of the level of the peat formation to tide-level, some 

 incursions of salt water, probably local in extent, may have taken place, and that 

 the effects of such immersions are causally connected with the structural char- 

 acters as well as the chemical qualities of the fuel. 



"It is, of course, well known that in many regions peat is in process of forma- 

 tion in salt marshes, that is, in coastal-inlet or estuarine swamps covered by salt 

 water at high tide. It does not, however, seem clear to the writer that coal with 

 so large a percentage of mother of coal, jetlike wood, etc., and with such pure 

 carbonaceous matter, that is, containing so moderate a percentage of ash, as the 

 coal in the Carboniferous of Illinois and Indiana, or that interbedded with marine 

 or brackish-water beds in Wyoming, was laid down in estuaries flooded by sea- 

 water at high tides or even at times of ordinary storms. Unfortunately, adequate 

 analyses for the determination of the ash in tidal-marsh peats from many localities 

 seem to be wanting. In this connection it must, however, be remembered that 

 the coal-forming jungle itself, by its great fecundity and profusion of growth, 

 and its correspondingly rapid contribution of refuse and fallen trees, may have 

 constituted an effective barrier into which the salt-water overwash of extra- 

 ordinary tides or storms would be able to penetrate only a relatively short distance 

 (perhaps a few miles) compared with the great area, often scores of miles in extent, 

 of the arborescent vegetal growth. Such an almost impenetrable vegetal barrier 

 would not only retard or dam the inrush of salt water pending its neutralization, 

 but also would quickly arrest the sediments and, according to the force of the rush, 

 retain them near the periphery of the swamps. It is not improbable that the 

 higher ash-content that is apt to characterize coal beds immediately overlain or 

 interbedded with marine sediments may result from occasional invasions of 

 brackish water into the swamps, the consequent death and decay of the fresh- 

 water types and, for short intervals, the deposition of peaty sediments higher in 

 ash, as is characteristic of brackish or salt-water peats at the present day. 



"Within the mouths of the estuaries and outlets of the lagoons, as well as 

 at the border of some of the swamps, there must have been transition zones in 

 which the water was at times more or less saline, but just which of the Paleozoic 

 plant types served in these frontier positions, maintaining their stand on the 

 border (brackish) zone of a salt-water habitat, is not at present fully known, 



