191-.] STEVENSON— THE FORMATION OF COAL BEDS. 553 



ancient trough of great subsidence, where deposits, throughout the 

 Palaeozoic, attained great thickness and whence they decrease quickly 

 toward the west. The assertion of greater altitude for the western 

 valley is based on absence of all deposits earlier than those of the 

 latest New River in the northern half of the area. 



Each basin had its longitudinal river. That of the east, rising 

 in the present confines of New York, flowed with low gradient for 

 more than i,ooo miles, receiving many tributaries from the bold 

 Appalachia and many, perhaps, unimportant tributaries from the 

 gentle slope at the west. Flowing at first close to Appalachia, it 

 was pressed constantly westward by alluvial fans and cones, which 

 became confluent and finally were modeled into a vast river plain. 

 The main stream was sluggish and often interrupted ; during high 

 floods, the surface was covered broadly by a sheet of water and the 

 debris from different streams was mingled. The river in the west- 

 ern basin received no debris-laden tributaries from east or west, 

 except at the extreme north ; it was more rapid than that in the 

 east and pushed its coarse materials far southward. Progressive 

 overlaps show that subsidence prevailed throughout the basin until 

 the later stages, when it was confined to the contracting area of 

 deposit ; but it was differential and not constant. There were long 

 intervals of slight or no movement during which rivers, reduced to 

 base-level, distributed mostly fine material along their lower reaches. 

 At the close of the Pottsville, the valleys had been filled and Alle- 

 ghania had become buried ; the whole area of deposit was an irregu- 

 lar marshy plain. But the old drainage systems continued until near 

 the close of the Conemaugh and determined the lines of sea invasion ; 

 they disappeared only with changes in the topography, induced by 

 the forces which were eventually to obliterate the basin. During 

 the whole of the Pennsylvanian, a very great part of the basin was 

 near sea-level. After the close of the Pottsville, few portions of the 

 area of deposition seem to have been more than 300 feet above tide 

 and there is no reason to suppose that any portion was at any time 

 much more than 100 feet below tide. 



The writer has become convinced that one must seek explanation 

 of the phenomena of the Appalachian basin in those of the great 

 river plains of modern times ; and the phenomena of the Appalachian 

 basin are those of coal regions elsewhere. 



