the delta and alluvial plain of
the Mississippi have been caused by the subsidence, rather than the
upheaval of land, appears to me established by the fact, that there are
no protuberances of upraised alluvial soil, projecting above the level
surface of the great plain. It is true that the gradual elevation of
that plain, by new accessions of matter, would tend to efface every
inequality derived from this source, but we might certainly have
expected to find more broken ground between the opposite bluffs, had
local upthrows of alluvial strata been of repeated occurrence.

_Antiquity of the delta._--The vast size of the alluvial plain both
above and below the head of the delta, or the branching off of the
uppermost arm of the Atchafalaya, has been already alluded to. Its
superficial dimensions, according to Mr. Forshey, exceed 30,000 square
miles, nearly half of which belong to the true delta. The deposits
consist partly of sand originally formed upon or near the banks of the
river, and its tributaries, partly of gravel, swept down the main
channel, of which the position has continually shifted, and partly of
fine mud slowly accumulated in the swamps. The farther we descend the
river towards its mouth, the finer becomes the texture of the sediment.
The whole alluvial formation, from the base of the delta upwards, slopes
with a very gentle inclination, rising about three inches in a mile from
the level of the sea at the Balize, to the height of about 200 feet in a
distance of about 800 miles.

That a large portion of this fluviatile deposit, together with the
fluvio-marine strata now in progress near the Balize, consists of mud
and sand with much vegetable matter intermixed, may be inferred from
what has been said of the abundance of drift trees floated down every
summer. These are seen matted together into a net-work around the
extensive mud banks at the extreme mouths of the river. Every one
acquainted with the geography of Louisiana is aware that the most
southern part of the delta forms a long narrow tongue of land protruding
for 50 miles into the Gulf of Mexico, at the end of which are numerous
channels of discharge. This singular promontory consists simply of the
river and its two low, flat banks, covered with reeds, young willows,
and poplars. Its appearance answers precisely to that of the banks far
in the interior, when nothing appears above water during inundations but
the higher part of the sloping glacis or ba