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Show 250 GROUPING OF STRATA IN DELTAS. two s1' d es by 1o f ty mounta. ins ' recei.v i.n g from them many rivers and torrents of different sizes, and If It be bounded on .the other st' de s, w1 1 e1.e t]1e surplus waters issue, by a comparat. ively low country, I' t I·S n ot .d ifficu. lt to define .s ome of the le. admg geo.} o. gi·C a1 1i!e a t ut. e s wllich will characterize the lacustrme fo. rmatiOn when this basin shall have been graduall~ converted mto dry l an d b yI·n f lux of fluviatile sediment. 'I he strata . w. ould1 be d. · 'bl · nto two principal groups; the olde'r compnsmg t 1ose IVISI e I • d' , . h . deposits which originated on the side a JOining t e mountams, where numerous deltas first began to form; and. the newer group con SI'st'1ng of beds deposited in the more central parts. of ·h b · and towards the side farthest from the mountams. t e a sin, h · · 1 k f The following characters w~uld form ~ e p~mcipa mar ~ o distinction between the strata In each series. The more anc1ent system would be composed, for the most part, of coarser mate: 1 ntainina many beds of pebbles and sand often of gt·eat na s, co l:l 'd bl 1 tl · kness and sometimes dipping at a cons1 era e ang e. TlhlCe se, •w ' ith associated beds of finer m• gred I' ents, wou Jd , 'f I traced round the borders of the ba~in, be seen to vary greatly j 11 colour and mineral composition, and would a~so be very irregular in thickness. The bed~, on the contrary, m th~ newer group, would consist of finer particles, and wo.uld be honzo?~al, or very slightly inclined. Their colour and mmcral compos1t10n would be very homogeneous throughout large areas, an~ would differ from almost all the separate beds in the older senes. . The following are the causes of the diversity ~ere allude~ to between the two great members of the lacustrme formatiOn. When the rivers and torrents first reach the edge of the lake, the detritus washed down by them from the adjoining heights ~inks at once into deep wate~·, all the heavier pebbles and sand subsidinoo near the shore. The finer mud is carri~d somewhat farther ~ut, but not to the distance of many miles, for the ·reater part may be seen, where the Rhone enters the Lake of ~eneva, to fall down in clouds to the bottom not far from the river's mouth. Certain alluvial tracts are soon formed ~t tie mouths of every torrent and river, and many of these, m th~ course of ages, become several miles in length· P~bbles a~n sand are then transported farther from the' JJiountams,. but their passage they decrease in size by attrition, and are m part .converted into mud and sand. At length some of tllc numerous GROUPING OF STRATA IN DELTAS. 251 deltas, which are all directed towards a common centre, approach near to each other-those of adjoining torrents become united and are merged, in their turn, in the delta of the larooest rive/ which advances most rapidly into the lake, and rend~rs all th; minor streams, one after the other, its tributaries. The various mineral ingredi~nts of each are thus blended together into one homogeneous mi~ture, and the sediment is poured out from a common channel Into the lake. As the average size of the transported particles decreases continually, so also the force and volu~e of the cune~t augments, and thus the newer deposits are chffused over a wider area, and are consequently more horizontal than the older. When there were many independent d~ltas near. the borders of the basin, their separate deposits differed ent1rel~ from each other. We may suppose that one was charged, hke the Arve where it joins the Rhone with whit~ sand and sediment, chiefly derived from decom~osed gramte-th~t another. was black, like many streams in the Tyrol, flowmg from Incoherent rocks of dark slate-that a ~hird ~~s coloured by ochreous sediment, like the Red River m Lomstana-and ~hat ~ fourt~, like the Elsa in 1,uscany, held much ~ar?onate of ~Ime m solutiOn. At first, they would each form ?Istmct deposits of sand, gravel, limestone, marl, or other matcnals; but after their junction, new chemical combinations and_ distinct colours would be the result, and the particles, havmg be~n con:eyed ten, twenty, or a greater numbet· of miles over alluvial plams, would become finer, . In deltas where the causes are more complicated, and where tides and currents. partiall~ interfere, the above description wo~tld only be apphcable, with certain modifications; but if a series of ea~thquakes accompany the growth of a delta, and change the levels of the land fmm time to time, as in the region whet·~ the Indus now enters the sea, and ·others hereafter to be me~tloned, the phenomena will then depart widely from the ordmary type If d . h · we possesse an accurate series of maps of t e Adri f £ . h . h a Ic or many t ousand years, our retrospect would ~lt out doubt, carry us gradually back to the time whe~ he number of rivers descending from the mountains into t at gulf b · d d Tl d I y In epen ent deltas, was far greater in number le e tas of the Po and the Adjge, for instance would sepa~ rate them 1 · h' ' se ves Wit m the ltuman era, as, in all probab.ility, |