198 PROBABLE RATE OF SUBSIDENCE. [CHAP. XXXIV. 



dermine them, and, aided by the river and tides, sweep much of 

 them away, and perhaps shape out a bay. But the swamp-mud, 

 with innumerable interlaced roots of cypress and other trees, might 

 offer considerable resistance ; and, after a time, the river charged 

 with sediment would throw down bars, and form a breakwater, 

 to protect the newly upraised deposits from annihilation. 



In regard to the time consumed in accomplishing the great 

 oscillation of level which first depressed so large an area to the 

 depth of 200 feet or more, and then restored it to its former po 

 sition, it is impossible, in the present state of science, to form more 

 than a conjecture as to the probable mean rate of movement. 

 To suppose an average sinking and upheaval of two and a half 

 feet in a century, might be sufficient, or would, perhaps, be too 

 great, judging from the mean rate of change in Scandinavia, 

 Greenland, the north of the Adriatic, and other regions. Even 

 such an oscillation, if simultaneously continuous over the whole 

 area, first in one direction, and then in another, and without any 

 interruptions or minor oscillations, would require sixteen thousand 

 years for its accomplishment. But the section at Cincinnati 

 seems to imply two oscillations, and there would probably be 

 pauses, and a stationary period, when the downward movement 

 ceased, and was not yet changed into an upward one. Nor 

 ought we to imagine that the whole space was always in motion 

 at once. 



When we have at length done our best to trace back the his 

 tory of the more modern and more ancient alluvial formations of 

 the Mississippi, the question still remains, what may be their age 

 relatively to the great body of the drift containing erratic blocks 

 in the northern latitudes of this same continent. The terraces 

 of gravel and loam bordering the Ohio, and those on a larger 

 scale, but of the same age, which constitute many of the eastern 

 bluffs of the Mississippi, are evidently features of subordinate im 

 portance in the physical configuration of the continent. But to 

 explain the origin of the northern drift of the Canadian lake dis 

 trict, and of the St. Lawrence, as I have endeavored to show in 

 my former &quot; Travels,&quot; requires a reference to such changes as 

 would imply the submergence of a great part of the continent 



