grown in their native forest.
(Tour beyond the Rocky Mountains, p. 132.) Some have inferred
from these facts that a tract of land, more than twenty miles in
length, must have subsided vertically; but Capt. Fremont, Dec.
1845 (Rep. of Explor. Exped. p. 195), satisfied himself that the
submerged forests have been formed by immense land-slides from
the mountains, which here closely shut in the river.

[365] For an account of the "sunk country," shaken by the
earthquake of 1811-12, see Lyell's Second Visit to the United
States, ch. 33.

[366] Darby's Louisiana, p. 103.

[367] The calculations here given were communicated to the
British Association, in a lecture which I delivered at
Southampton in September, 1846. (See Athenaeum Journal, Sept.
26, 1846, and Report of British Association, 1846, p. 117.)
Dr. Riddell has since repeated his experiments on the quantity
of sediment in the river at New Orleans without any material
variation in the results.

Mr. Forshey, in a memoir on the Physics of the Mississippi,
published in 1850, adopts Dr. Riddell's estimate for the
quantity of mud, but takes 447,199 cubic feet per second as
the average discharge of water for the year at Carrolton, nine
miles above New Orleans, a result deduced from thirty years of
observations. This being one-tenth more than I had assumed,
would add a tenth to the sediment, and would diminish by
one-eleventh the number of years required to accomplish the
task above alluded to. "The cubic contents of sedimentary
matter," says Forshey, "are equal to 4,083,333,333, and this
sediment would annually cover twelve miles square one foot
deep."

[368] The Mississippi is continually shifting its course in
the great alluvial plain, cutting frequently to the depth of
100, and even sometimes to the depth of 250 feet. As the old
channels become afterwards filled up, or in a great degree
obliterated, this excavation alone must have given a
considerable depth to the basin, which receives the alluvial
deposit, and subsidences like those accompanying the
earthquake of New Madrid in 1811-12 may have given still more
depth.

[369] Account of the Ganges and Burrampooter rivers, by Major