244 ON THE DISTRIBUTION OF FRESH WATER FISHES 



rapids. It is probable that such obstructions to the east and westwai-d flowing rivers 

 may have existed for a long time where they pass the Allegheny ridges, but the 

 breaches which they selected could only have been results of the greatest of the 

 denuding agencies which have existed in that region. 



Such an elevation of waters would not, however, have caused a communica,tion of 

 the head streams of the Holston and Kanawha, nor indeed of any of the four rivers, 

 especially as it is probable that at that time they were separated by elevations some- 

 what greater than now. 



Of the three submergences here indicated, it is not possible that of the modified 

 drift should have been so deep as to have reduced mountains of 10,000 and 20,000 

 feet to their present elevation ; otherwise its deposit over the country would have 

 been moi'e extensive. The submergence during which the tertiaries of Bra"ndon and 

 Monte Alto were deposited could not have been profound enough for this result, 

 otherwise these beds would not contain fruits and other remains of shore plants so 

 abundantly. As we have no indications of any other submergences, the period of 

 denudation may most probably have been that during which the Alleghenies slowly 

 rose from the bed of the carboniferous ocean ; for deposits made at such a period have 

 at least had time for removal and reappearance in our extensive cretaceous and neo- 

 comian beds of the east. 



Hence no destruction of the fishes since that period can, be asserted to have been 

 caused by submergences. 



The destruction of the river fauna cannot be accounted for on the supposition of 

 the poisoning of the waters by sulphhyric acid, vitriol, alum, or other waters. The 

 thermal and mineral springs of Virginia have, in former periods of greater activity, 

 no doubt, poisoned many streams, but not probably all those of any one basin at 

 one time. Therefore it is improbable that any one species of fish could have been 

 destroyed in this way. The occurrence of greater cold, and more extensive deposits 

 of ice and snow during the glacial epoch of the north, could only have driven fishes 

 to lower waters. So an opportunity seems to have offered for a continuous " descent 

 with modification," as^has been already suggested in the first section. 



We may then turn to another means of communication. The subcarboniferous and 

 Devonian limestones, which form the beds of the valleys in this region and portions 

 of the bases of the mountains, are everywhere penetrated by caverns, and in many 

 places traversed by faults. Now it appears quite possible that the waters of one 

 stream, in some elevated part of their course, might occasionally have sunk into a 

 fault, or penetrated the limestone on one side of a monoclinal mountain, and so have 

 found its way into the opposite valley of a stream of another hydrographic basin, 

 carrying with it several species of fishes. That the upper waters were chiefly con- 

 cerned in this commingling is indicated by the fishes which are common to any two 



