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Show DltiFT TIMBE1t OF THE MACl{ENZIE. [Ch.XV. whole firmly together. Sections of these islan~s .ar~ annu~lly made by the river, assisted by the frost; and It lS mterest~ng to study the diversity of appearances they present accordmg to their different ages. The trunks of the trees gradually decay until they are converted into a blackish brown substance resembling peat, but which still retains more or less of the fibrous structure of the wood ; and layers of this often alternate with layers of clay and sand, the ~hole being penetrated, to the depth of four or five yards or more, by the long fibrous roots of the willows. A deposition of this kind, with the aid of a little infiltration of bituminous matter, would produce an excellent imitation of coal, with vegetable impressions of the willow roots. What appeared most remarkable was the horizontal slaty structure that the older alluvial banks pre. sented, or the regular curve that the strata assumed from unequal subsidence. " It was in the rivers only that we could observe sections of these deposits, but the same operation goes on on a much more magnificent scale in the lakes. A shoal of many miles in extent is formed on the south side of Athabasca Lake, by the drift-timber and vegetable debris brought down by the Elk River; and the Slave Lake itself must in process of time be filled up by the matters daily conveyed into it from SlaveRiver. Vast quantities of drift timber are buried under the sand at the mouth of the river, and enormous piles of it are accumulated on the shores of every part of the lake*:" The banks of the Mackenzie display almost everywhere horizontal beds of wood coal, alternating with bituminous clay, gravel, sand, and friable sandstone; sections, in short, of such deposits as are now evidently forming at the bottom of the lakes which it traverses. Notwithstanding the vast forests intercepted by the lakes, a still greater mass of drift-wood is found where the Mackenzie reaches the sea, in a latitude where no wood grows at present except a few stunted willows. At the mouths of the river the ~ Dr. Richardson's Geo~nost. Obs. on Capt. Franklin's Polar Expedition, Ch. XV.] IMDEDDINO OF TERRESTIUAL PLANTS, ~43 alluvial matter has formed a b arn.e r o f I.s 1a n d s an d sh oals where we may expe t .c • ' . c a great wrmatwn of coal at some distant penod. T. he abundanc. e of floating timber on the Mac k enzt.'e . IS owmg, as I am mformed by Dr. Richardson t th ]' . . ' o e peen 1ar du·ectwn and to the lenath of the course f th' · · o o 1s nver, wh1ch ~·uns from south to north, so that the sources of the stream lie m much warmer latitudes than its mouths · I n t h e country therefore, where the former are situated the f· t b k ' • • , 1 os rea s up at an earhcr s.e ason, while yet the waters in the 1o wer part of I· ts course are Ice-bound. Hence the current of water, rush 'm g down northward, reaches a point where th e tl1 aw h as not begun, and finding the channel of the river blocked · 1 · · fl up Wlt 1 Ice, It ove.r ows the banks ' sweeping through f,o res t s of pm· es,· and carrymg away thousands of uprooted trees. :', e ~a~e. already observed* that the navigation of the M1ss~ss1ppt Is much impeded by trunks of trees half sunk in the ~1ver. On rea.ching the Gulf of Mexico many of them subside and are Imbedded in the new st1.a t a w h1' c h f,o rm the delta, but many of them float on and enter the Gulf-stream. '~Tropic~! plants, (says M. Constant Prevost,) are taken u.p by tlus great current, and carried in a northerly direction, till they reach the shores of Iceland and Spitzbergen uninjured. A great portion of them are doubtless arrested on their passage, and, probably, always in the same inlets, or the same spots on the .bottom of the ocean; in fact, wh~rever an eddy or calm determmes their distribution, which, in this single example, ext~nd~ over a space comprehended between the equatm· a.nd the eightieth degree of latitude-an immense space, six times ~ore. considerable than that occupied by all Europe, and thu-ty t1mes larger than France. The drifting of various ~ubsta?ces, though regular, is not continual; it takes place by mtermittance after great inundations of rivers, and in the in-tervals the wat . 1 eis may on y carry sand_ or mud, or each of these alternately, to the same localities t." . * Vol. i. p. 245. t Mem, de la Soc. d'Hist. Nat, de Pal·is, vol. iv:p. 84• R2 |