86 



found its more perfect development in the region now occu- 

 pied by the Genesee River. 



Eventually .shallow, brackish, or nearly fresh water condi- 

 tions returned, and the black shales, which succeed the 

 gray, were deposited. These deposits became more and 

 more sandy as time progressed, and eventually culminated 

 in more or less argillaceous sandstones. The faunas of the 

 Genesee shales returned in a more or less modified condition, 

 fishes being especially prominent. As the shoaling of the 

 water continued, sands were deposited exclusively, and so 

 the thick beds of Chemung sandstones were formed, suc- 

 ceeded later by beds of subcarboniferous conglomerate. 

 Then the mud making conditions, which had at intervals 

 occurred, and during the existence of which the carbonaceous 

 shales were deposited, returned with greater perfection and 

 greater permanency. Continuing long, they permitted a 

 luxurient growth of vegetation, which, becoming buried in 

 detrital deposits, has given us the Pennsylvania coal-beds. 

 Similar conditions existed in other regions, where beds of 

 coal also accumulated. The Pennsylvania coal-beds may be 

 regarded as the record of the consummation of that coal- 

 making tendency, which was so continuously exhibited 

 during the middle and later Devonian, in this region, but 

 which, at no time preceding the coal-measure ( meso-carbonic ) 

 period, produced results which were in any way comparable 

 to those produced during these later ages. 



The sandstone beds of the Chemung period were probably 

 the last beds to be spread over the Erie County region, 

 which shortly after the commencement of the Carbonic era 

 became dry land. The shore-line was transferred to some- 

 where in the vicinity of the boundary line between Xew York 

 and Pennsylvania, nearly all the area comprised within the 

 latter state being under water. It was in this great bay-like 

 indentation, and in another one which stretched north into 

 Michigan, that the chief coal-beds were deposited.* 



♦Dana in the 4th Edition of his Manual of Geology, gives on p. 633 an instructive 



map of the- outlines of the land at the beginning of the Carbonic era. 



