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Show l86 TRANSPORTATION OF MATTER f the Ohio the plain is from thirty to :fi~ty mi~es broad, and 0 '. t 't on increasing in w1dth till the expanse after that pom 1 goes d f h' h f as great 1 On the bor ers o t Is vast is perhaps t rec Imcs . · " bl ffs " as the are alluvial tract are perpendicular cliffs, or u ' y d d f limestone and other rocks. For a great calle compose o l ff: ,, d b 1 dista;ce the Mississippi washes the eastern "~ u s ; an . c ow h f th Ollio never once comes m contact With the the mout o e ' 'd b western. The waters are thrown to the eastern s1 e, dechausc all the large tributary rivers enter from the. west, an ave filled that side of the great valley with a slopmg mass ?f clay d d F tlll·s reason the eastern bluffs are contmually an san . or · ' · 1 un d ermm. e d , an d the Mississippi is slowly but mcessant y pro-gressing eastward :l(: • • Tl1 e ri·v er t rav erses the plain in a meandermg cour.s e, d 'bi'ng 1'mtnense and uniform curve:::;. After sweepmg roeuSCnfdl the half of a circle, it is preci• p•i tated f rom t hc pom' t in a current diagonally across its own channel, . to another curve of the same uniformity upon the opposite shor~ 1'· Tl1 ese curves are So reoO 'ular ' that the boatmen and Indians ca l cu 1a t e distances by them. Opposite to each ·o f thf e·m , 1t'h ere is always a sand-bar, answering, in the convexity o It.s J.Orm, to the concavity of " the bend," as it is called t. Th: river, by continually wearinO' these curves deeper, returns, hke many other streams befo:e described, on its own tract, so .that a.vess:I in some places, after sailing for twenty-five or thirty miles, ~s brought round again to within a mile of the place whence ~t started. When the waters approach so near to each other, It often happens at high floods that they burst ~hrough the small tongue of land; and, having insulated a port!on, rush through what is called the "cut off" with great. velocity. At one. spot called the "grand cut off," vessels now pass from one pom.t to another in half a mile, to a distance wl1ich it formerly requ~rcd twenty miles to reach §. After the flood season, ~hen the river subsides within its channel, it acts with destructive force upon the alluvial banks, softened and diluted by the recent overflow. Several acres at a time, thickly covered with wood, are precipitated into the stream; and the islands formed by the pro· • Geograph. Descrip. of the State of Louisiana, by W. Darby, Philadelphia, 1816, p. 102. . L · 154 t Flint's Geog., vol. i., P• 152. t Ibid, § lb1d., vo 1 ·' P· · BY TilE MISSISSIPPI. 187 cess before described, lose large portions of their outer circumference. " Some years ago," observes Captain Hall, " when the Mississippi was regularly surveyed, all its islands were numbered, from the confluence of the Missouri to the sea; but every season makes such revolutions, not only in the number but in the magnitude and situation of these islands that this enumeration is now almost obsolete. Sometime~ large islands are entirely melted away-at other places they have attached themselves to the main shore, or, which is the more correct statement, the interval has been filled up by myriads of logs cemented together by mud and rubbish·*.'' When the Mississippi and many of its great tributaries overflow their banks, the waters, being no longer borne down by the main current, and becoming impeded amongst the trees and bushes, deposit the sediment of mud and sand with which they. are abundantly charged. Islands arrest the progress of floatmg trees, and they become in this manner reunited to the land; the rafts of trees, together with mud, constituting at length a solid mass. The coarser portion subsides first, and the most copious deposition is found near the banks where the soil is most sandy. Finer particles are found at th? farthest distances from the river, where an impalpable mixture is deposited, forming a stiff unctuous black soil. Hence the alJuvions of these rivers are highest directly on the banks, and slope back like a natural " glacis" towards the rocky cliffs bounding the great valley t. The Mississippi, the~efore, by the continual shifting of its course, sweeps away, durmg a great portion of the year, considerable tracts of alluvium which were gradually accumulated by the overflow of former years, and the matter now left during the springfloods will be at some future time removed. One of the most interesting features in this basin is '' the raft." ;r'he dimensions of this mass of timber were given by Darby, In 1816, as ten miles in length, about two hundred and twenty yards wide, and eight feet deep, the whole of which had accumulated, in consequence of some obstruction, during "' Travels in North America, vol. iii., p. 361. t Flint's Geography, vol. i., p. 151. |