6 ORIGINAL CONNECTION OF THE EASTERN AND 



continued over the Cincinnati axis, would have required not far from 

 one million of years. No geologist who has attentively considered the 

 evidences of duration given us by many geological facts can doubt that 

 far more than this time has elapsed since the beginning of the tertiary 

 period. The best computations of the duration of time represented 

 by that period, — those made by Mr. James Croll, — assign a duration 

 of over four million years from the beginning of the eocene to the 

 present day. It would require not more than half this time to take 

 away a section which, if restored to this district, would give us the 

 drainage surface within the beds above the level of the conglomerate. 

 Without setting too much value on the estimates of the duration of the 

 tertiary period, we are certainly safe in saying that the time that has 

 elapsed since the close of the carboniferous period must much exceed five 

 millions of years; yet this period, probably but a small part of the age 

 that has elapsed since our coal-beds were laid down, is sufficient, at the 

 present rate of erosion, to have taken away something like fifteen hun- 

 dred feet of strata from this region. It is therefore necessary for us to 

 suppose that the cai'boniferous strata originally extended over this region ; 

 or else Ave must arbitrarily, and without a trace of affirmative evidence, 

 suppose that Avhile the carboniferous series was not deposited in this 

 region, the trias, the Jurassic, or the cretaceous beds were laid down, 

 and since their deposition utterly wasted away, leaving no debris to 

 mark their former occurrence in these parts. One or the other of these 

 suppositions is necessary. The geological reader can safely be left to 

 choose between them. 



"While I regard the physical evidence of the original extension of the 

 carboniferous deposits across the upper Green river district from the 

 eastern to the western fields as practically complete, it is necessary to 

 add that it cannot be inferred from this that the whole of the Cincinnati 

 axis was so covered by the formations of the carboniferous series. As is 

 evident from a mere inspection of the Cincinnati axis, the two extremities 

 of the ridge are geologically much higher than the intervening district. 

 The amount of the central deflection is several hundred feet, and it is 

 through it that we have complete proof of the former connection of the 

 two coal-fields by a coal-bearing belt of not less than fifty miles in width. 

 The question of the former extension of the coal over the Cincinnati and 

 Nashville ends of the axis must remain, for the present, an open ques- 



