CHAP. XXXIV.] PEOBABLE RATE OF SUBSIDENCE. 263 



apparently identical with the South American species; 

 remains of the mastodon, elephant, ox, and other 

 mammalia, much resembling, on the whole, those 

 found at Natchez and on the Ohio. 



As to the seaward extremity of the ancient delta, 

 the effect of the gradual depression of land above 

 assumed would be to cause its mud and sand to 

 increase in thickness, instead of augmenting in area. 

 When at length the movement was reversed, and 

 the freshwater deposits began to rise, the action of 

 the sea would undermine them, and, aided by the 

 river and tides, sweep much of them away, and per 

 haps shape out a bay. But the swamp-mud, with 

 innumerable interlaced roots of cypress and oiher 

 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 position, it is im 

 possible, 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 up 

 heaval 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, Green 

 land, 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 



