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Show 222 DE L1'A OJt 'l'llg ItllONg the shallowing of the bottom began to be very sensible at a c1 .1 s t ance of ca l,out a mile and three-quarters• from the mouth of the llhone; for a line drawn from St. Gmg~ulph to Vevcy, : mean depth of somewhat less than s1x hundred feet, gtves a fl . .1 d . l and from that part to the Rhone, the uv1atl emu 1s a ways ~ d lonO' the bottom*. We may state, therefore, that the !OUn a b · · strata annually produced arc about two m1les m length: so that, notwithstanding the great depth of the .lake, the new l 'ts are not inclined at a high angle; the chp of the beds, c epos1 d . d' 1• 110J Ce d ' I·S so slirrht that they would be terme , 111 or mary b ' L • geo1 o g1· c.a llanguaOb 'e ' horizontal. The stra• ta probably c• ons1sts of alternations of finer and coarser particles, for during the hottet months from April to August, when the snows melt, the volume and velocity of the river are greates.t, and large quantities of sand, mud, vegetable matter, and d.l'lft w~od are I·n tr·o d L1 ced ·, but, during· the rest of the year, the mflux Is co•m - paratively feeble, so much so, that the whole lake, accor~mg to Saussure, stands six feet lower. If then, we could obtai? a section of the accnmulatiou formed in the last eight ~entul'l.cs, we should see a O'reat series of st1•ata, probably from SIX to nme hundred feet thkk, and nearly two miles in length, inclined at a very s1ight angle. In the mean time, a great numbet· of small~r deltas are growing aroun.d the bor?ers of the luke, at the mouths of rapid torrents, whiCh pour 111 large mas~es of sand and pebbles. The body of water in thes~ torrents 1s too small to enable them to spread out the .,transported matter over HO eXtensive an area as the llhone, rf bus, fo~ example, there is a depth of eighty fathoms within half a .mile of the shore, immediately opposite the gt·eat torrent. whwh enters. east of Ripaille, 80 that the dip of the strata m that .del~a ts about four times as great as those deposited by the mam nver at the upper extremity of the lake t. . . The capacity of this basin being now as?ert~med, It would. be an interesting subject of inquiry, to dete~mme m what number f1 years the Leman Lake will be converted mto dry land. It w~u not be very difficult to obtain the element~ for su~h a calc~l~twn; so as to approximate at least to the quantity of t1me req~Ired fo f the accomplishment of this result. The number of cubic feet o * De la Beebe, M.S. t Ibid, IN TIU~ LAKE OF GENEVA. 223 water annually dischm·ged by the river into the lake beinoestimated, experiments might be made in the winter and sum~ met· months, to determi11e the proportion of matter held in suspension or in chemical solution by the Rhone. It would be also necessary to ~llow ~or the heavier matter drifted along at. th.e bottom, wlnch might. be estimated on hydrostatical prmciples,. when the average s1ze of the gravel and the volume and velocity of the stream at different seasons were known. Supposing all these observations to have been made, it would be more easy to calculate the future than the former proO'J·ess of the delta, because it would be a laborious task to asce1~ain with any degree of precision, the original depth and extent of ~hat par~ of the lake which is already filled up. Even if this mformatwn were ac~uratcly. ob.tained by borings, it would only enable .us to approximate w1tlnn a certain number of centuries to.the time when the Rhone began to form its present delta; but tl118 would not give us the date of the oriO'in of the Leman Lake • . b m Its present form, because the river may have :flowed into it for thousands of years, without importing any sediment whatever. Such would ha:e been the case, if the waters had first passed through a cham of upper lakes; and that this was actually the fact, is indicated both by the course of the Rhone bet ween Martigny and the Lake of Geneva, and still more decidedly, by the channels of many of its principal feeders. If we ascend, for example, the valley through which the Dranse flows, we find that it consists of a succession of basins, one abov~ the other, in each of which there is a wide expanse of flat alluvial la~ds, separated from the next basin by a rocky gorge, once evidently the barrier of a lake. The river has ~lle~11the lake, and partially cut through the barrier, which it I~ st1 gradually eroding to a greater depth. The examinatiOn of almost all valleys in mountainous districts affords abundant proofs of the obliteration of a series of lakes hv the ?lling up of hollows and the cutting through of r~cky barners-a process by which running water ever labours to produce a more uniform declivity. Before, therefore, we can pretend ('Ven to hazard a conjecture as to the era at which any ~articul~r delta commenced, we must be thoroughly a?qnamted with the geological history of the whole system of higher valleys which communicate with the main stream, and all |
