204 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



"With the Ames limestone, inroads of the sea practically ceased. Marine 

 conditions unquestionably were repeated, but never for periods long enough for 

 good development of animal invertebrate life. Limestones appear frequently 

 during the upper half of Conemaugh, several of them widely, though irregularly, 

 distributed, but in no case are they distinctly marine. Some are crowded with 

 minute univalves of undetermined relations; others are associated with carbon- 

 aceous shales, filled with fragments of plants and fishes, which point rather to 

 fresh-water conditions. 



"The most notable feature of the Conemaugh is the red and green shales, in 

 color resembling those of the Catskill and Shenango, but deeper. The greater 

 development is in west central West Virginia and the adjacent part of Ohio, 

 where at times nearly the whole section is red shale. The greatest geographical 

 expansion was just preceding the deposition of the Ames limestone, when the reds 

 reached southeast nearly to the outcrop and northward to the outcrop in Penn- 

 sylvania; but they did not reach into northern Ohio and they are practically 

 wanting east from the line of Chestnut Hill in Pennsylvania. From that time 

 to the end of Conemaugh the area contracted and reds occur in irregular patches. 

 These beds frequently contain nodules of limestone, and are usually fossiliferous. 

 The red shales in some cases mark horizons elsewhere carrying limestone, and 

 they may indicate a marine condition. 



"The exceeding shallowness of the water and the long periods of quiet during 

 the Conemaugh are indicated by the coal beds, which, though extremely thin, 

 have great extent. * * * 



"Toward the close of the Conemaugh the streams bringing in materials had 

 become sluggish, and the deposits, e.vcept within limited areas, are fine in grain. 

 The Monongahela began with a long period of exceedingly slow subsidence, 

 during which the Pittsburg coal bed gradually extended across the northern part 

 of the great basin and southward along the east and west sides; but from all 

 sides it became thinner toward the central part of the basin and it is practically 

 wanting in a great part of West Virginia and eastern Ohio, where it occurs only 

 in widely separated patches. The bed may have been almost continuous around 

 the basin. The singular uniformity of conditions and the extreme slowness of 

 movement are shown by the structure of this great bed, persisting in such minute 

 details as partings in tracts of thousands of miles and reappearing even in isolated 

 patches within West Virginia. 



"The area of greatest subsidence during the Monongahela did not coincide 

 with that of the earlier formations, as appears abundantly from comparison of 

 sections along several lines. The deepest deposits of Allegheny and Conemaugh 

 were at the north and east; not so in the Monongahela. * * * The greatest 

 subsidence was in north-central West Virginia, whence the thickness decreases in 

 all directions. 



"With this change in place of chief subsidence there came clearly a farther 

 contraction of the basin, while elevation at the north led to spreading out of 

 sandstone along much of the northern border. This Pittsburgh sandstone is not 

 present in the eastern localities of Pennsylvania and Maryland, but is persistent 

 in the Chestnut Ridge area of Fayette and Westmoreland Counties, in that State, 

 as well as southward along the eastern outcrop in West Virginia to the last 

 exposure near Charleston, where Doctor White found it 70 feet thick. Evidently 

 it prevailed along the western outcrop in Ohio, for it is present on the north- 

 western outcrop and also in the central counties along that line, where one is 



