194 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



shales or limestones, although no distinctly marine faunas have been observed, 

 thus indicating for those basins estuarine waters, which at times may have been 

 completely isolated, though still in close geographic relation to the maiine. 

 Brackish- water deposits are present at one or more horizons in most of the 

 extensive coal-bearing formations, whether younger or older, even those which 

 for the most part contain only fresh-water types of life.^ Usually, however, 

 the formations are apt also at certain levels to carry distinctly marine faunas. 

 The parallelism of the strata and the frequency of the salt-water invasions show 

 the constant nearness to tide-level, whereas the horizontal extent of the strata 

 indicates the areal extent of the region of deposition. It may therefore be 

 confidently stated that in most of the great coal fields of the world the coal was 

 formed on a nearly level surface at or near sea or lake level. 



"Coastal Swamps. 



"The enormous horizontal extent of many of the coal groups as indicated 

 by the remnants now found in basins detached as the result of folding and erosion, 

 the demonstrated continuity of some of the individual coal beds over areas some 

 of amazing size, the high degree of parallelism of the beds, and the recognition 

 that the coal beds were laid down beneath a water cover, join in predicating the 

 existence, at the time the coals were being formed, of vast swamps and broad 

 but shallow lagoonal areas subject to subsidence at a generally slow rate. Such 

 coastal or lacustrine swamps, of magnitudes unknown in the world of to-day, 

 appear to have resulted either from the partial submergence of a very mature 

 and broadly extended surface of erosion (peneplain) or from so extensive and so 

 nearly a complete filling of great parts of a basin as to develop, under the action 

 of waves and currents, extremely shallow littoral flats many miles in width. 

 Following periods of deeper subsidence and the deposition of varying thicknesses 

 of other materials, such as clays, sands, and calcareous muds, there was recurrence 

 of proximate filling of the loaded areas of the basin, and occasionally there was 

 slight uplift of the region and consequent withdrawal of the water, so that the 

 level of the subaqueous deposits was so near to the surface of the water cover as 

 again to favor swamp conditions. Sometimes there was sufficient elevation of 

 the region and retreat of the water to bring the old bottom some distance above 

 the water level of the basin; but it is probable that actual elevation to any con- 

 siderable distance above the sea was exceptional and confined in most cases to 

 the intervals of more marked earth-movements separating many of the geologic 

 formations. Conditions of relative quiescence and of partial exposure must 

 have taken place many times during the deposition of series of strata embracing 

 many coal beds. The periods of such proximation of sea-level and land-level 

 were times of extension of the vascular plant cover to occupy the shallow regions, 

 thus initiating the formation of a new coal bed. If the slope of the exposed land 

 was reduced to a sufficiently low gradient and the rainfall was heavy and well 

 distributed, the swamp conditions would have been extended far inland, the 

 drainage of the water being held back, obstructed, by the most luxuriant and 

 fecund plant growth, so that the state of partial submergence of the land would 

 have been carried for indefinite distances along the nearly flat land surface. At 

 such times the true coast-line was doubtless locally obliterated, except in so far 

 as barrier beaches marked the zone of wave-action and bordered the expanses 

 of open water. 



^ For example consult the stratigraphic sections of the great coal fields of Europe and America. 



