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Show CHAPTER XIII. Repro d uc t1. ve ef fie c t s of running water-Division of deltas into lacustrine, modi-terranean, and oceanic-Lake deltas-Growth of the delta of the Rhone in the Lake of Geneva-Chronological computations of the age of deltas- R t d 't · Lnke StliJcrior-Deltas of inland seas-Rapitl shallowing t~CCil cpOSl S 111 "' " • , ~ of the Baltic-Arguments for and against the hypothes1s of Celsms~Elevated beaches 011 the coast of Sweden-Marine delta of the Rhone-V ~nous proofs of its increase-Stony nature of its deposits-Delta of the Po, Ad1ge, Isonzo, and other rivers entering the Adriatic-Rapid conversion of that gulf into land-Mineral characters of the new deposits-Delta of the Nile-Its increase since the time of Homer-Its growth why checked at present. WE have hitherto considered the destroying agency of running water, as exhibited in the disintegration of rocks and ~ransportation of matter from higher to lower levels. lt remams for us to examine the reproductive effects of the ~ame ~ause .. The aO'O'regate amount of matter accumulated m a g1ven time at tl~; mouths of rivers, where they enter lakes or se~s, affords clearer data for estimating the energy of the excavatmg power of running water on the land, than the separate. stud! of.the operations of the same cause in the cou~tles~ ~am1fic~t10ns mto which every great system of valle!s Is divided. We shall proceed to select some of the leadmg facts at present ascertained respecting the growth of deltas, a~d shall then offer some general observations on the quantity of subaqueo.us sediment transported by rivers, and on the manner of .Its distribution. Deltas may be divided into, first, those winch are formed in lakes ; secondly, those formed in inland se~s ; and thirdly, those formed on the borders of the ocean. 'I. he most characteristic distinction between the lacustrine ~nd mar~ne deltas consists in the nature of the organic remams, whwh become imbedded in their deposits; for, in the case of a lake, it is obvious that these must consist exclusively of such genera of animals as inhabit the land or the waters of a river or lake; whereas in the other case, there will be an admixture and most frequen;ly a predominance of animals which inha?it salt wat~. In regard, however, to the c}istribution of inorgamc matter, t e LAKE OF GENEVA, 221 deposits of. lakes and inland seas are formed under very analogous circumstances, and may be contm-distinguished from thos.e on t!1e shores of the great ocean, where the tides co-operatmg with currents give rise to a distinct class of phenomena. In lakes and inland seas, even of the largest dimensions, the tides are almost insensible, and the currents are, for t~lC most ~art, inc~nsiderable, although some striking exceptJOns to this rule will be mentioned when we tt·ent of tides and currents. DELTAS IN LAKES. Lalce of Geneva.-It is natural to begin our examination with an inquiry into the new deposits in lakes, as they exemplify the first reproductive operations in which rivers are engaged when they convey the detritus of rocks and the ingredients of mineral springs from mountainous regions. 'l'he accession of new land at the mouth of the Rhone, at the upper end of the Lake of Geneva, or the Leman Lake, presents us with an example of a considerable thickness of strata, which have accumulated since the historical era. This sheet o. water is about thirty-seven miles long, and its breadth is from two to eight miles. The shape of the bottom is very irregular, the depth l1aving been found, by late measurements, to vary from twenty to one hundred and sixty fathoms *. The Rhone, where it enters at the upper end, is turbid and discoJoUt ·ed; but its waters, where it issues at the town of Geneva, are beautifully clear and transparent. An ancient town, called Port Vallais, (Portus Valesire of the Romans,) once situated at the water's edge, at the upper end, is now more than a mile and a ~Ja]finland,-this intervening alluvial tracthavingbeenacquired m about eight centuries. The remainder of the delta consists of a flat alluvial plain, about five or six miles in length, composed of sand and mud, a little raised above the level of the river, and full of marshes. Mt· · De la Bee he found, after numerous soundings in all parts of the lake, that there was a pretty uniform depth of from one hundred and twenty to one hundred and sixty fathoms throughout the central region, and, on approaching the delta, • De lnBeche, Ed. Phil. Journ., vol. ii., p. 1071 Jan. 1820 |