X9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 427 



except on the western edge, where it is brought up by a great anti- 

 cline and is lo to 15 feet thick, while, at 4 or 5 miles farther west 

 in Fayette county, it has disappeared and the Upper P'ocono beds of 

 the Mississippian rest on the Chemung. The rate of decrease under 

 Somerset is very nearly the same as that in Fulton and Bedford. 

 This thinning is shown on the western side of the Catskill deposits 

 from New York to New River in Virginia, beyond which south- 

 wardly Chemung and Catskill both disappear.^ 



The Carboniferous was opened by subsidence in the basin. The 

 northern portion still received the greatest deposits along the eastern 

 side in the old valley or " trough of sedimentation," but the area 

 widened westwardly so that the later Pocono rocks of Pennsylvania 

 overlap the Catskill of the Devonian and rest without apparent non- 

 conformity on the Chemung rocks of Alleghania, as they do beyond 

 in the Ohio basin. There was distinct widening eastwardly in 

 Virginia and southward. CampbelP showed that phenomena in 

 Virginia, which had puzzled earlier observers, were due to overlap ; 

 that the coal-bearing Mississippian deposits rest there on Ordovician 

 rocks, which in all probability had been upraised during the 

 Taconian revolution. Still farther south, the oldest rocks of the 

 Mississippian overlap the Catskill, the Chemung and, at length, even 

 the thin Chattanooga shales, the last Devonian representative toward 

 the south. But during succeeding stages of the Mississippian, 

 there was distinct contraction of the area of deposit on the western 

 side, for the Maxville lies within the Logan and the Shenango within 

 the Maxville f at times, there may have been dry land in the Alle- 

 ghania region. But at the south there was continued depression, 



*J. J. Stevenson, "Bedford and Fulton Counties,"' Second Geo!. Surv. 

 of Penn., 1882, pp. 73-75, 81 ; " The Upper Devonian Rocks of Southwest 

 Pennsylvania," Amer. Jour. Set., III., Vol. XV., 1878, pp. 423-430; "On the 

 Use of the Name Catskill," ibid., Vol. XLVI.. 1893. pp. 330-337. 



^ M. R. Campbell, " Palaeozoic Overlaps in Montgomery and Pulaski 

 Counties, Virginia," Bull. Geol. Soc. Amer., Vol. S, 1894, p. 182. 



*J. J. Stevenson, "Lower Carboniferous of the Appalachian Basin," 

 Bull. Geol. Soc. Amer., Vol. 14, 1903, p. 85. This paper, on pp. 89-96, con- 

 tains a discussion of the varying geographical conditions during the Missis- 

 sippian. 



