INTERPRETATION OF ENVIRONMENTAL CONDITIONS 197 



"The prevalence of thick, smooth barks on the trees is another indication 

 of a humid swamp environment. * * * 



"Proof that the Stigmaria roots (which are known to have belonged to Sigil- 

 laria and several other related lycopodialean types) have in most cases grown 

 in the clays where they are now found beneath the coal or in the clay partings, 

 may generally on close examination be found in (a) the radial position of the inter- 

 lacing rootlets passing outward from the parent root in all directions and oblique 

 to the bedding; (h) the penetration of buried pieces of stem, bark, or other partly 

 decayed roots by the rootlets of later growth, a common occurrence; (c) the fact 

 that the roots extend outward from the base in a normal manner and are "right 

 side up," not having been disturbed or overturned. 



"Coal Formation in Brackish or Marine Waters. 



"The question arises as to the extent to which coal of the common types may 

 have been laid in brackish or marine waters. This problem is particularly 

 prominent in certain Paleozoic coal fields like those of the Interior basins, in which 

 many coal beds are capped by black shales, generally containing marine shells. 

 The occurrence of Stigmaria roots in place of growth in the underclays of the 

 Carboniferous coal of Missouri, Indiana, and Illinois, even where, as in many 

 places in Missouri, the underclays rest on thin limestones of marine origin, or the 

 occurrence of roots of other kinds beneath the Cretaceous and Tertiary coal, 

 including most of those interbedded in marine formations, must be interpreted as 

 indicating either subaerial conditions previous to an initial stage of inundation 

 of the old soil or a situation at the initial stage of coal formation in which the 

 water was shallow enough to permit rooting and growth of a dense coal-forming 

 vegetation in place. In the latter case it becomes probable that here and there 

 more elevated parts of the bottom, such as shoals, bars, or barrier beaches and 

 broader barriers, the latter often the result of slight warping of the strata of the 

 region, rose above the water-level and excluded the sea from free access during 

 most, if not quite all, of the period of peat deposition. It may further be as- 

 sumed — and observations of the bedding of the ordinary coal fully support this 

 assumption — that barriers of these kinds were numerous, and that they divided 

 the coal field into smaller areas, which in most cases were very numerous and 

 irregular in form. The latter feature is, of course, normal to the tide-level pene- 

 plain surface. 



"The physiographic conditions, including the advanced degree of filling of the 

 basins, the perfection of the base-level, the somewhat unequal loading, and the 

 recognized slight warping of the basin floors, all support the hypothesis that 

 during the times of coal formation the areas of broad and unbroken sea expanse 

 were very restricted, being probably confined in most cases to the deeper parts 

 of the basin, 1 now usually concealed. That, however, the sea probably did in 

 some cases break over the coal-formation swamp is indicated by the intercalation 

 of over-washed muds, silts, or sands, forming partings few of which contain 

 brackish or salt water shells such as Lingula and Orbiculoides. Other overwashes 



^ It is not improbable that such an area of deeper water, largely without appreciable coal 

 deposition, occupies the axis of the Appalachian trough, as defined in Monongahela and 

 Dunkard time, in southwestern Pennsylvania and northern West Virginia. In the 

 first-mentioned State the absence of the coal and the reduction in thickness of the 

 lower coal formations is proven by abundant drillings. Farther south, on the West 

 Virginia side of the valley, especially where the younger Pennsylvanian formations rise 

 to daylight, the changed character of the formations is well recognized. 



