150 J. J. STEVENSON CAKBONIFEEOUS OF APPALACHIAN BASIN 



northern border in Ohio, may be evidence of slight elevation on that side, 

 and the pebbles' may have been derived from ui3raised Eockcastle beds. 



The existence of land at the sonth and southvrest is indicated by the 

 distribution of the coal beds. These appear to be persistent around the 

 borders of the basin, even at the south and southwest, to the last localities 

 where one may recognize the formation; but they are wanting in the 

 interior area within Ohio and West Virginia, where in thousands of 

 square miles no trace of them appears in the well records. 



At the close of the Beaver, though the whole basin had received de- 

 posits, the water was very shallow and the surface of the latest sandstone 

 or shale showed surprisingly little irregularity. The abruptness of 

 change in many localities from Homewood sandstone to fine shale of the 

 Allegheny is one of the most notable conditions. Not less remarkable 

 is the regularity of the subsidence in a vast area. The interval from 

 Homewood sandstone to the Brpokville (Stockton) coal bed varies little 

 from 10 feet in a space of more than 1,000 square miles within "West 

 Virginia, while in other tracts equally extensive that interval varies from 

 5 to 20 feet. Whatever may have been the cause, a long period of very 

 gentle change succeeded the Beaver, during which coal at the Brookville 

 horizon, with greatest thickness at the east, gradually spread over the 

 greater part of the area in wliich Allegheny rocks remain. Its extent is 

 unequaled by that of any deposit at a later horizon. 



The Allegheny is a thin formation, but its variations in thickness are 

 considerable. Along the easterly side one finds 160 feet in Broad Top 

 of Pennsylvania, 350 to 260 in the Potomac area, decreasing toward the 

 west; 175 in Taylor coimty of West Virginia, beyond which it decreases 

 to 140 feet, a thickness which is retained for many miles. Increasing 

 again, it becomes 200, and at the Kentucky border about 210 feet, which 

 seems to be approximately the thickness assigned to its beds in south- 

 eastern Kentuclvy by Mr Hodge. In the First and Second basins of 

 Pennsylvania the measurements vary from 235 to 275, but along the. 

 west side of Chestnut hill it is 230 in Indiana county of Pennsylvania, 

 206 at the West Virginia line and 195 feet at Clarksburg, about 40 miles 

 south from that line. The greatest thickness is in Jefferson, Armstrong, 

 and Butler counties of Pennsylvania, becoming 340 feet in the last, 

 whence it decreases toward the west and is barely 200 feet at the Ohio 

 line. In that state the variation is between 175 and 240 feet, the least 

 thickness being on the western border, whence it thickens eastwardly 

 toward the middle of the basin, where in the deep portion of Ohio and 

 West Virginia it is about 250 feet. There seems to be no reason for 

 supposing that the Allegheny becomes thicker southward in Kentucky, 



