128 ACCOUNT OP THE 



been cut down by currents, and it would, 1 think, be more 

 rational to regard them as the signs and products of a for- 

 cible elevation of that quarter, or else as cracl^s from the 

 drying of the strata after being left by the sea, or even 

 as inequalities formed simultaneously with the deposition of 

 the strata where they are found. At some period subse- 

 quent to the complete retirement of the waters of the ocean, a 

 third era in the formation of our valleys seems to have com- 

 menced. The rivers became much reduced in size, and 

 began to flow in narrow channels, supported by the alluvion 

 which they had previously transported. From this time, 

 they appear to have effected but little horizontal detrition. 

 They have increased the depth of their beds, but not widened 

 their valleys. The Ohio, as you may see by a reference to 

 the Chart, has a channel below the bottom of the alluvial 

 deposits, to which it is confined, when not swollen by rains. 

 This, I apprehend, is its own worl<, and has been effected 

 since the existing state of the surface of the continent was 

 produced. In front of Cincinnati, the depth of this canal is 

 between thirty and forty feet in the centre, and it becomes 

 shallower as we ai)proach either sliore, apparently at the 

 same rate at which the velocity of the current decreases, 

 from the middle to the margins of the stream. This bed is 

 doubtless becoming deeper ; but the ratio of increase is ex- 

 tremely slow, as no abrasion takes place exxept when the 

 momentum of the waters is augmented by floods, which are 

 generally transient in their duration. At all other times, the 

 velocity of the current at the bottom is a minimum, or ap- 

 proaching to it. This is obvious in summer and autumn, 

 when the water is so low and transparent, that its bottom 

 may be seen from the surface. It may then l)e observed, 

 that the bed in many places, from side to side, is covered 

 with rolled pebbles ; and that the rocks, where not thus pro- 

 tected, are overspread with nunute algw 7nusci, dead leaves, 

 twigs, and other light bodies, whicli the gentlest current 

 wotdd sweep away. Now where there is no current, it is 

 obvious there can be no detrition. 



