INTERPRETATION OF ENVIRONMENTAL CONDITIONS 195 



"VARYING SUBSIDENCE. 



"The thickness, and even the survival, of the peat bed thus begun depended 

 upon several conditions, important among which are contemporaneous subsidence 

 and its rate. It is probable that at times the water body was greatly contracted, 

 receding to deeper and perhaps relatively small areas of the basins; and at other 

 times accelerated regional subsidence caused the reextension of the water-level 

 over the great border zone (littoral). Variation in the rates of subsidence are 

 well established and can be observed even in the present epoch. At times of more 

 rapid depression, when the water became too deep to be interrupted by bars or 

 shoals or when the water-level in the basins was too high and too expansive for 

 subaerial plants to root and grow on the bottom, the water was open to movement 

 and clay and sand were distributed by the waves and currents, or perhaps lime- 

 stone was formed or calcareous mud was laid down. The effect of these oscillatory 

 movements accompanied by the leveling of the sediments in the basin, as shown 

 by the studies of such extensive areas of continuous horizontal beds as are seen 

 in the Appalachian and the Interior coal fields, seems to have been the production 

 at various times of enormous expanses of broad, shallow pans or lagoons, over the 

 greater part of whose areas the water was not too deep for occupation by the 

 dense and luxuriant vegetation of the time. The examination of the stratigraphy 

 of the Appalachian coal fields and of the Interior basins shows that during the 

 periods of deposition of the coal-bearing groups the filling of the broad, shallow 

 Carboniferous basins nearly kept pace, on the whole, with the subsidence, which 

 obviously varied in rapidity; but the water was never really deep (probably less 

 than 200 feet in the deepest axes, except at rare periods of greatest subsidence), 

 even in the relatively restricted central areas of the basin, far from the varying 

 coast-lines. 



"FORMATION OF BARS AND BARRIERS. 



"At other times great areas, probably embracing most of the depositional 

 region, were either above water or so near the surface that sand barriers or bars, 

 possibly in series, and probably more or less irregular in plan, developed far out 

 toward the edge of the submerged shelf, constituting series of lagoonal or land- 

 locked shallows of enormous aggregate extent. Where not too deep or subject to 

 vigorous incursions of the sea these were occupied by the coal-forming vegetation. 

 It is also apparent that in parts of the Appalachian trough large areas of the 

 submerged flats were, for variable periods, isolated from other parts of the basins 

 by differential movements or warpings of the basin, which produced barriers or 

 low arches of great linear extent, these barriers having greater magnitude and 

 permanency than the bars or other lesser barriers just mentioned. Against the 

 bars, barriers, and shoals that appear to have separated the pans or great swamp 

 expanses, the coal beds usually pinch out, though it is frequently observed that, 

 as the result of continued subsidence, or, in cases, as the result of the impounding 

 of water by the dense vegetal growth, which, in a humid climate, raised the 

 ground-water level, the peat bridged many of the shoals and bars. 



"It is more than probable that during these periods of coal formation in the 

 broadly extended swamps and lagoons of fresh water the very luxuriant and 

 intricate tangle of peat-producing vegetation, much of it of large size, that occu- 

 pied the shallow and protected areas, effectually obstructed the penetration to 

 any great distance into the swamps, of salt water, and rendered the peat-forming 

 areas practically nonmarine a few miles back of their seaward margin, though 

 portions of these areas may have been at sea-level. Such obstruction would be 



