CHAP. XXXIV.] OSCILLATION OF LEVEL. 195 



ulating, however, on the manner in which the valleys of the Mis 

 sissippi and its tributaries may have been affected by subterranean 

 movements, we are at least authorized by analogy to assume that 

 the downward movement may have been greater in the more 

 inland part of the continent, just as we have seen in 181112, 

 that the &quot; sunk country&quot; west of New Madrid subsided, while 

 the level of the delta at New Orleans underwent no sensible- 

 change. If, then, the vertical movement in the interior, in and 

 near the valley of the Ohio, for example, were greater than near 

 the Gulf, as, if. in the former case, it were two and a half feet in 

 a century, and near the sea only half that amount, it would fol 

 low that the general fall of the rivers would be lessened. They 

 would deposit all their heavier, and some even of their finer sedi 

 ment, in their channels, instead of having power to carry it to the 

 sea. They would fill up their beds, and often overflow the ad 

 joining plains, raising their level by repeated layers of fluviatile 

 matter or silt, frequently containing the shells of land and amphib 

 ious mollusks. 



If, even now, the Mississippi, when flooded, dams up the mouths 

 of its great tributaries, and transforms them for months into tem 

 porary lakes, it must have produced the same effect to a far greater 

 extent if at any time the general fall of the country toward the 

 sea was less rapid. 



In narrow valleys bounded by ancient rocks 500 or 600 feet 

 high, such as that of the Ohio, the alluvial formation could never 

 acquire great breadth. Its thickness would depend entirely on 

 the length of time throughout which the subsidence was prolong 

 ed. But nearer the sea, where the continent falls with a gentle 

 slope toward the Gulf, the encroachment of the fresh-water de 

 posits (No. 2, fig. 11, p. 196), of the great river on the tertiary 

 strata (No. 3), constituting the original bluffs on its eastern and 

 western boundaries, might be very great. 



If we then suppose the downward movement to cease, and to 

 be at length converted into an ascending one, the rate of up 

 heaval being greatest in the more inland country, the fall of every 

 river, and consequently its velocity, would begin immediately to 

 augment. Their power of carrying earthy matter seaward, and 



