INTERPRETATION OF ENVIRONMENTAL CONDITIONS 203 



stones, though extending over a great part of the basin at the north, are either non- 

 fossiliferous or contain only fresh- water forms ; but south from the Ohio River in 

 Kentucky the upper Freeport limestone carries a characteristic Carboniferous 

 fauna. 



"It is wholly probable that the Appalachian and the Indiana- Illinois fields 

 were not united during the Allegheny, though they may have been during the 

 Rockcastle, as they were during the Mississippian." 



C. INTERPRETATION OF CONDITIONS IN CONEMAUGH 

 AND DUNKARD TIME. 



Mr. Stevenson here (page 154) notes the possible occurrence of persis- 

 tently aggrading streams from the Homewood sandstone stage to the, in places, 

 base of the Lower Freeport: 



"Toward the close of the Allegheny a small area in west central West Virginia 

 near the Ohio River received deposits of red mud, more or iess calcareous, accom- 

 panied often by greenish muds; and, somewhat earlier, similar deposits were 

 made in northeastern Kentucky. This is the beginning of a condition which in 

 gradually enlarging or contracting area was to continue until the close of Carbon- 

 iferous time, always predominating, however, within a small area in West Virginia 

 and the adjoining part of Ohio. 



"In many respects the Conemaugh is but the continuation of the Allegheny; 

 the variations in thickness are, geographically, very similar in both. * * * Except 

 in a very narrow strip along the southeasterly border in West Virginia, the Cone- 

 maugh sandstones are more irregular than are those of the Allegheny. One 

 generally finds some sandstone of some sort in the sandstone intervals, but shales 

 predominate in by far the greater part of the area. * * * Away from the south- 

 eastern border, pebbles are extremely rare, except along a narrow rudely east- 

 and-west strip across Indiana, Armstrong, Butler, Lawrence, and Beaver counties 

 of Pennsylvania. This lies many miles south from the northern outcrop and 

 south from the similar strip in the Beaver formation; its variations are such as 

 one finds in the gravels of the upper Ohio River. Many similar valleys filled 

 with standstone during the long subsidence are recognizable in various parts of 

 the area, and occasionally one is found along an anticlinal crest which seems to 

 have been made by subaerial erosion. The sandstones for the most part are 

 indefinite within Ohio, but in Tuscarawas County the Lower Mahoning interval 

 filled with conglomerate and farther south the Buffalo interval is filled with very 

 coarse sandstone at many places. * * * 



"While in a general way the conditions were similar to those of the Allegheny, 

 showing a gradually contracting area, yet the subsidence was such as to admit 

 sea-water to a much greater space. At the very beginning one finds at somewhat 

 widely separated localities in West Virginia a marine fauna in the Uffington shale 

 which rests directly on the Upper Freeport coal bed, while at most exposures the 

 shale yields only impressions of land plants. Not enough information is avail- 

 able to justify any suggestion respecting the relations of the marine localities, 

 which are confined to the easterly side of the great basin." [It is here suggested 

 by Mr. Stevenson that the Brush Creek limestone is due to an invasion from the 

 east, while the Cambridge limestone is due to an invasion by the Mississippian 

 sea from the west.] 



