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Show 234 DELTA. OF TilE RHONE, Mediterranean driven by winds from the s~utb, sand-bars are often formed across the mouths of the r1ver: by these means considerable spaces become divid~d o~ fro~ the sea, and subsequently from the river also, when 1t sh1fts Its channels of efflux. As some of these etangs, as they are called, are subject to the occasional ingress of the river when flooded, and of the sea during storms, they are alternately salt and fresh. Others, after being filled with salt-water, are often lower~d by evaporation till they become more s~lt than the s~a.; and It has happened, occasionally, that a considerable preCipitate of ~uriate of soda has taken place in these natural saltct:ns. Durmg the latter part of Napoleon's career, w.hen the excise-laws were enforced with extreme ricrour, the pohce was employed to prevent such salt from being ~sed. The fluviatile and marine shells enclosed in these small lakes, often live together in brackish water; but the uncongenial · nature of the fluid usually produces n dwarfish size, and sometimes gives rise to strange varieties in form and colour. Captain Smyth, in the late survey of the coast of the Mediterranean found the sea, opposite the mouth of the Rhone, to dcepe~ gradually from four to forty fatho~s, within a distance of six or seven miles, over which the discoloured freshwater extends; so that the inclination of the new deposit~ must be too slicrht to be appreciable in such an extent of sectiOn as a gcologi~t usually obtains in examining ancie~t formations. When the wind blew from the south-west, the shtps employed in the survey were obliged to quit their moorings; and when they returned, the new sand-banks in the. delta were foun~ covet·ed ovet· with a great abundauce of mar~ne shells.. By tins means, we learn how occasional beds of dnfted marme shells may become interstratified with fresh-water strata at the mouths of rivers. . . That a great proportion, at least, of the new dep~sit m the delta of the Rhone consists of rock, and not of loose mcoherent matter, is perfectly ascertained. In the museum at Montpellier is a cannon taken up fro~ the sea near the mouth. of the river, imbedded in a crystallmc calcareous rock. Laige masses also, are continually taken up of an arenaceous rock, cement'e d by calcareous matter, includ·m g mu 1t l· tu d es of broke.n shells of recent species. The observations recently made on this DELTA OF TilE PO. 235 subject corroborate the former statement of Marsilli *, that the earthy deposi~s of the coast of Languedoc form a stony substance, for whiCh reason he asct·ibed a certain bituminous saline an d g l utm. ous nature, to the substances brought dow' n with' sa.nd by the Rhone: If t?e num~er of mineral springs charged with carbonate of hme which fall mto the Rhone and its feeders in different parts of France be considered, we shall feel no surprise at the lapidification of the newly-deposited sediment in this delta. It should be remembered, that the fresh-water introduced by rivers, being lighter than the water of the sea floats . ' over the latter, and remams upon the surface for a considerable distance. Consequently, it is exposed to as much evaporation as the waters of a lake; and the area over which the riverwater is spread, at t?e j m~ction of great rivers and the sea, may well be compared, In pomt of extent, to that of considerable lakes. Now, it is well known, that so great is the quantity of water carried off by evaporation in some lakes, that it is nearly equal to the water flowing in ; and in some inland seas, as the Caspi.an, it is quite equal. We may, therefore, well suppose that, m cases where a strong current does not interfere, the greater portion not only of the matter held mechanically in suspension, but of that also which is in chemical solution, must be precipitated within the limits of the delta. When these finer ingredients are extremely small in quantity, they may only s~fficc to supply crust~ceous animals, corals, and marine plants, With the earthy particles necessary for their secretions; but ~hene~er it is in excess (as genera1ly happens if the basin of a r1;e1· he. partly i? a district of active or extinct volcanos), then Will sohd deposits be formed, and the shells will at once be included in a rocky mass. . Delta o( the Po.-The Adriatic presents a great combination of circumstances favourable to the rapid formation of deltas-a gulf receding far into the land,-a sea without tides or stl'ong currents, and the influx of two rrreat rivers the Po and the Adige, besides numerous minor s~reams dr:ining on the one side a great crescent of the Alps, and on the other * Hist. Phys. de Ia Mer. |
