90 J. J. STKVENSON- 



very thin, but persisting for considerable distances. The massive struc- 

 ture and great thickness characterize the Logan under cover in south- 

 eastern Ohio and eastward to Roane county of West Virginia, where it 

 is replaced by the shales to which reference has been made. In south- 

 ern Ohio, liowever, the coarseness diminishes, and in the upper Knob- 

 stone of Kentucky one finds only fine grained sandstone, which in turn 

 grows finer and more shaly until in Tennessee the Lauderdale character 

 is assumed, which prevails in western middle Tennessee and in north- 

 western Alabama. There is, then, a sandstone deposit crossing the 

 northern portion of the basin, extending southward almost to the central 

 line of West Virginia ; on the west side this extends southward into 

 Tennessee, but on the east side hardly any farther than in West Virginia ; 

 the south central portion of the basin almost to Tennessee is occupied 

 by shales. From both sides the deposit grows finer toward the central 

 line of the basin. 



The form of the area of sedimentation differs from that of the latest 

 Devonian. It seems hardly possible that the Logan extended northward 

 into New York except at the extreme northeast; its limit in northern 

 Ohio is far south from that of the Cuyahoga, while its western limit in 

 that state could hardly have been very far beyond the present line of 

 outcrops ; but southwardly the area widened until beyond the Cincin- 

 nati peninsula it became broadly continuous with sediments of the 

 Mississippi area. On the eastern side the boundary can have been little 

 different from that of the Devonian as far south as the Tennessee line. 

 Southward the latest Devonian overlaps the Catskill by many miles 

 and is in turn overlapped by the Logan, which reaches into central 

 Alabama, overlapping even the Chattanooga black shale, which may 

 represent the Hamilton of New York. The southward expansion of the 

 area was very gradual ; for while Kinderhook, Burlington, and Keokuk 

 occur in northwest Pennsylvania and Ohio, the lower members disap- 

 pear southwardly in succession, so that in northwestern Alabama only 

 the Keokuk appears to be present. The rate of subsidence must have been 

 practically the same in by far the greater part of the area, contrasting 

 in this respect notably with prior times, for from Cambrian almost to 

 the close of the Devonian a great trough of subsidence lay along the old 

 shoreline at the east. 



The source of material found along the westerly outcrop is ver}^ un- 

 certain. The rocks of earlier periods on that side are fine grained, very 

 largely calcareous or argillaceous; even the Berea sandstone is very 

 fine ; yet for a long distance the Logan is largely conglomerate and the 

 sandy deposit continues much farther southward on the west than on 

 the east side. Tlie i)ebbles of quartz are small and flat, as though chafed 



