WESTERN COAL-FIELDS OF THE OHIO VALLEY. 9 



centre and summit. This is clearly the view that is most reconcilable 

 with the present conditions and the record of ancient conditions which 

 is sent down to us in the physical record of the rocks. 



The biological evidence which we may derive from the rocks about the 

 Cincinnati axis does not favor the idea of its having been a barrier during 

 any stage of the carboniferous time, from the base of the Waverley to the 

 highest coal-bearing strata. The only level where we find evidence of its 

 having been a distinct bai'rier is in the time between the upper Cincin- 

 nati beds and the base of the Black Shale. During this time, when the 

 upper hundred feet of the Cincinnati series was depositing, and during the 

 whole of the Niagara and Corniferous periods, the Cincinnati axis gives 

 us evidences that it was a distinct ridge, rising to or above the surface of 

 the sea; but from that time down to the last of our records of the ancient 

 seas it appears to have been always merged in the uniform oceans or 

 swamps of those days. The fossils of the subcarbonifcrous period do not, 

 so far as I have been able to examine them, indicate shore-lines along 

 this axis. It is true they differ on the two sides of its central line, but 

 this difference seems to me to indicate the steady deepening of the sea 

 from its eastward shore-line towards what is now the centre of the Mis- 

 sissippi Valley. 



The conclusions which I believe we are forced to accept from the evi- 

 dence the rocks afford may be briefly summed up as follows: 



1st. That the Cincinnati axis Avas about the level of the sea during a 

 part of the Hudson River, Medina, and Niagara eiiochs. 



2d. That during the subsequent ages, down to and including the carbon- 

 iferous series, the axis was probably of no importance as a physical or 

 zoological barrier. 



3d. That the coal-period swamps, and the seas into which they from 

 time to time sank down to receive their burial in the drifting sands and 

 muds, extended over the most if not the whole of this axis. 



A study of the evidences of a former connection of the eastern and 

 western coal basins in Kentucky affords us some data for estimating the 

 former extension of these deposits in the other parts of the Ohio Valley. 



It is clear that an erosion scarcely more considerable than that which 

 has taken place in Kentucky would have sufficed to separate the basin of 

 the Appalachian region from that of Michigan. 



