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Show 252 GROUPING OF DHPOSITS IN DELTAS. 1 f tl e Isonzo an d tl1 e Torre · If' on the other han· d, would t 1ose o 1 1 t n future c1 1 anges, we may anticip. a.t e the penod we specu a e 1 0 f d It will greatly dimimsh ; for t 1e when the t numn~temru eo to ee ncarso acll at the rate of a mile in a Po canna ndc oo ther n.v er.s t o g ain as much i.n six. or seven ce. n-century, a 11 lf without new JUnctiOns occurrmg turies ·u pon thte' sha owl gtuE, 'danus " the kinO' of rivers," will so t 1a • 1'1 ' e from. tim1e1 tob aImste a, great er nu mber· of tributaries. The. G.a nges contmua Y 0 d B oter I1 ave p1.0 b ab ly become confluent withm . the an nrrampo d 1 d t of the junction of the Ued River historical era; an . t le lal e. allli'kelihood have been known, h M, ' ipp1 WOU < , Ill ' • a. nd t e .· Is1st sds not b een so recently discovered. The umon If Ame'In,.c a. · la d t 11 e E~ up1 n a tes must undoubtedly have been of thef 11 gnsm aond ern· geogr.a p1 n ·c a l changes on our ea. rth, and one o t te . 1 t b extended to many other regwns. similar remarks nugfl l e M .'t'me Alps between Toulon and AI tl e base o t 1 e an 1 ' • ong 1 . . · tl f exceptions are now formmg strata Genoa, the n vcrs, w~ 1 e~ , Their channels are often several of conglomerate an sal~ ~~ m being dry' and the rest easily mnes in breadth, s?me 0 t1le . the year· whereas during the d i' ·ly eiO'ht mon 1s m ' . fordc. 101f' n1e al w otl 1 ey are swo lien ' ai1d a great transportatiO.n meltmg o t le sno k 1 In order to keep open the mam of mud and pebbles ta esl P ace. carried alon()' the sea-coast, it road from France to Ita y' ~~wgreat masses of shingle, brought is necessary to remove annua y A portion of the pebbles are down during the fl~ou-season. N'. to form beds of shingle . I r tieS as near Ice, seen m some oca l ' reater art are swept into a deep se~. along the shore, but th~ g b th P deltas of minor rivers on this The small Pl:ogress l:la e yhene we recollect that there is somecoast need not surpnse us, w d £ t t a few hundred yards from times a depth of two. thousa? nil:; o~servations might be made the beach, as near Nice .. Su f the rivers in Sicily, and, among · 1 ·ge pro.nortwn o 1 t f respectmg a at -r 1 . 1 . diately north oft 1C por o others, respecting that w llC l, Imme f granitic pebbles into Messina, hurries annually vast masses o the sea. . . (J' many mouths converge, a When the deltas of nvers havbmo h fluence of some one . £ t t k s place y t e con Partial umon at rs a e . . t tl'l the main trunks are or more o f t1 l ei· r ann8. .' but 1t IS no un d lt that a comp 1e t e h 1 d f the common e a, connected above t e. le~ . o d sediment takes place. intermixture of their JOlllt waters an 5TRATIFlCATION OF DEPOSI'rS IN DELTAS, 253 The union, therefore, of the Po and Adige, and of the Ganges and Burrampooter, is still incomplete. If we reflect on the geographical extent of surface drained by rivers such as now entet· the Bay of Bengal, and then consider how complete the blending together of the greater part of their transported matter has already become, and throughout how vast a delta it is spread by numerous arms, we no longer feel so much surprise at the area occupied by some ancient formations of homogeneous mineral composition. But our surprise will be still farther lessened when we afterwards inquire into the action of tides and currents, in disseminating the matter accumulated in various deltas. Stratification of Deposits in Deltas.-That the matter carried by rivers into seas and lakes is not thrown in confused and promiscuous heaps, but is spread out far and wide along the bottom, is well ascertained ; and that it must for the most part be divided into distinct strata, may in part be infetTed where it cannot be proved by observation. 'l'he horizontal arrangement of the strata, when laid open to the depth of twenty or thirty feet in the delta of the Ganges and in that of the Mississippi, is alluded to by many writers ; and the same disposition is we1l known to obtain in all modern deposits of Jakes and estuaries. Natural divisions are often occasioned by the interval of time which separates annually the deposition of matter during the periodical rains, or melting of the snow upon the mountains. The deposit of each year acquires some degree of consistency before that of the succeeding year is superimposed. A variety of circumstances also give rise annually to slight variations in colour, fineness of the particles, and other charactet·s. Alternations of strata distinct in textu1·e, mineral ingredients, or organic contents, are produced by numerous causes. Thus, for example, at one period of the year, drift wood may be carried down, and at another mud, as was before stated to be the case in the delta of the Mississippi ; or at one time when the volume and velocity of the stream are greatest, pebbles and sand may be spread over a certain area, over which, when the waters are low, £ne matter or chemical precipitates are formed. During inun<lations the current of fresh-water often repels the sea for many miles; but when the river is low, salt-water again occupies the same space, "\Vhen |