Ill 



ALLUVIUM. 



ALLUVII.M. 



is* 



k of flat, fertile, alluvial Und, with a gravelly bottom, the gradual 

 creation of th stream. Sometimes the ooune of the river in no tortuous 

 that two points, A and m, may be within a few hundred yard* of each 

 other, and yet, following the line of the stream, they may be tome 

 mile* arander. In this owe, the narrow neck of Und is acted ujioii 

 doubly ; for the force of the wnU-r in directed against it on each xi.li-. 

 In time thin isthmus u breachod, and the river either flown entirely 

 through the new channel, or, ilivi.ling, fornui the Und A into an inland. 



Such tortuous courses, when they are cut through solid rock, a> in 

 the ease of the Moselle, whom bank* are sometimes 600 feet high, are 

 among the strongest proofti of the destructive power of running water, 

 fir ii. i sudden deluge, however powerful, could Imvo scooped out such 

 ii trough ; and th.it a i-lcft of Hiich n nature Bhould be occs/riom-d by 

 any disruption nf the earth'* crust, in not lens improbable. More 

 midden and therefore more (hiking instances of the waste of the land 

 occur whore a river flows through a lake, and by its wanting action 

 causes a breaking-down of the barrier. We hare already alliul.il to 

 tin- 1. uniting of a lake in the valley of Ragnes in Switzerland. That 

 flood was produced ly the melting of ice, which, falling in successive 

 seasons from neighbouring glaciers, hnd formed no continuous a mass 

 M to dam up tlie water of a stream which flowed in the bottom !' tin- 

 valley. If the barrier of a lake consist of strata of rock, supported by 

 beds of clay or sand, and if, by any change of circumstance*, tho 

 miming water get access to this inferior bed, and gradually \va~u 

 it nway, tho superincumbent rock, thus undermined, suddenly breaks 

 down, and devastation and niin overwhelm the country below. 



The distance to which the .let ach.-d fragments are carried depends 

 upon the volume of water, and tin- nature of the ground over which it 

 flows. The torrent* from the south-western Alps, rushing over a steep 

 uninterrupted slope, transport large blocks to the sea; but a river 

 tlmt runs through a long stretch of level country deposits the grosser 

 matter in the upper part of its course, and carries to its mouth only 

 that which is more easily held in suspension. The larger stones, after 

 being detached from their parent rock, have therefore to undergo an 

 intermediate process of abrasion, by being nibbed against each other 

 in tli,! bed of the stream before their particles are finally committed to 

 the deep. If a river pass through a lake in its course, the solid matt. -r 

 will be deposited in that trough until it has filled it up ; and if the 

 lake be very large, even the lighter particles will have time to fall, 

 and the water will flow out clear from the other extremity. The 

 1,-ike of Geneva affords a remarkable instance of this process ; for the 

 Idiom-, where it enters, is extremely turbid ; but at Geneva, where it 

 leaves the lake, it is beautifully transparent. At the upper end then- 

 is :i tract of alluvial land nearly 8 miles in length, which has been 

 gradually formed by the deposits from the river; ami some measure 

 "t it" progress is obtained by the change in the situation of the town 

 of I'orl Vallais, which was onoe at the water's edge, but in the course 

 of about 800 years has been left a mile and a half inland. Other 

 torrents, on both sides of the lake, likewise pour in large quantities of 

 solid matter ; and thus, although from its great depth a long period 

 must eUpse if the present order of nature remains undisturbed, the 

 Lemon I*ake will be converted into green meadows, and cattle will 

 graze where there are now 160 fathoms of water. Nor is this an 

 extravagant expectation, or more than has taken place elsewhere in 

 past times. The vast fertile valley between the Vosges Mountains 

 and those of the Block Forest, through which the Rhine flows for 

 above 100 miles, between Straaburg and Worms, without falling more 

 than two feet in a mile, is in great part covered with alluvium, ami is 

 Idled to an unknown depth under the soil with sand and gravel 

 similar to that now transported by tin- Ithone. There is every reason 

 to believe that this valley was at one time the site of a lake far larger 

 tli. in that of Geneva, and probably quite as deep. 



The Ilhinc, in the higher port of its course, is filling up the Lake 

 of Constance, where a considerable tract of alluvial land has been 

 formed ; and, after issuing pure from the lower end, it appears from 

 the observations of Hammer to have carried on the work of destruc- 

 tion so powerfully in the comparatively short distance between the 

 Lake of Constance and the bottom of the falls at Schaffhausen, as to 

 have supplied materials sufficient to fill up several lakes between 

 ScbafThausen and Strasburg, besides the great lake below Strasburg 

 already spoken of. There are numerous instances of this gradual 

 filling up of lakes, especially in the courses of the greater rivers, as in 

 the Danube between Ulm and Keuburg above Vienna, and most 

 eminently so in the case of the St.-Lawrcnce. Simond state* thai 

 the river Lint, in Switzerland, is perpetually filling up its old channel, 

 .111-1 overflowing into a new one, in consequence of the man of 

 rubbish and stones brought down from the Ularus Mountains ; and 

 that the level of the Lake of Wallenstadt had been actually raised 10 

 f.-<-t in the previous 00 years by this accumulation. If tho river does 

 not meet with lake* in its course, and flows over a great extent of 

 e. Hiiitry having a slight degree of inclination, the transported matter 

 very often *o accumulates as to raise the bed of the stream itself. 

 One of the most striking instances of this kind is afforded by th-- I'", 

 the eoniuioii rec.ptuele of the waters of the numbcrli-ss hit-rent* 

 whirh rush down on both sides of it, loaded with spoils from tlm 

 Alps and Northern Apennines. The effect of this has been tlmt the 

 river has frequently shifted its course; and, to prevent the damage 

 that ensues from such events, the inhabitants of Lombardy have 



protected their lands by embankments, which confine the river to its 

 channel. This, however, U a work of inoenant labour, and deceptive 

 security, for the accumulation of matter in the bed goes on with 

 unremitting constancy; and, t-. prevent the water from overflowing, 

 the matter must be taken from the bottom and thrown upon the 

 banks, sometimes as much as a foot in a Mason. Tho effect of this 

 has been, that in the lower parts of ita course the ]'.. runs on t> 

 of a high mound, which even overtops the houses in Fen-are. 



In a mountainous country where the land rises rapidly from the 

 shore, the rivers descending over a steep bed sweep all the CM- 

 into the sea. If the neighbouring sea be deep, and the tides be si 

 an estuary or inlet is formed at the mouth of the river- that 

 sea forms a deep indentation int the I. in. I. of a triangular shape, 

 forming what -1 other geographers have fancifully < 

 ' negative delta.' If, on the other h.m.l. a low shelving : li.,i.-. an.! tin- 

 absence of strong tidal current* favour the gradual and tranquil deposit 

 of the solid matter brought down by the river, an extensive level of 

 alluvial land is formed. In this case the main river, at a distant ;...int 

 inland, often divides itself into two streams, which, gradually cli\ 

 until they reach the sea, inclose a triangular space of lain I 

 form of the fourth letter of the Greek alphabet, A, and hence called a 

 drlta. The man of water does not, however, long continue divided 

 into two streams only ; the jirocess of separation is repeated several 

 times, and thus the delta is traversed by several channels, and the 

 great river empties itself into the sea by many mouths, as may be seen 

 by tin- inxjM-ction of the Nile and Ganges in any map of Egypt or 

 Hindustan on a tolerably large scale. In this way a delta is f. .nue.l at 

 the mouths of the Rhine, Khdne, Fo, Daniilx-, \\ Indus, 

 Ganges, Orinoco, and mam others. The magnitu.l. of the delta. 

 generally, although not always, corresponds to the volun f tin- 

 waters by which it has been created. The head of that of the 1< Urn- 

 is about 90 miles distant from the general line of sea-coast of Holland ; 

 and although the name of the main river is almost lost by tl - 

 division of it waters and the. junction of other rivers, we ii 

 within the Uhine delta the whole of tho low-land from the ncighliour- 

 hood of Calais to the north-eastern slfores of the Xuydcr Xee, which 

 makes the bom of the triangle nearly 200 miles. The head of t he ilelt a 

 of the (hinges is 220 miles from the sea, its base is 200 mil. 

 including the space occupied by the two great arms of the ( 

 which bound it on either side. The tract in the lower part of this 

 delta, called the Sundei bunds, a wilderness infested by tigers and 

 diles, is, according to Kennell, equal in extent to the principality ,,f 

 Wales. The whole of a deposit within a delta, as well as much above 

 and on eaeh side of it, is therefore an encroachment of the land upon 

 the sea, and in many rivers this growth of the land is in a steady pro 

 gress of advancement; as, for example, the city of Kavenim, formerly 

 a seaport of the Adriatic, is now 4 miles inland. There are causes, 

 however, which often prevent the farther increase of a delta . 

 has advanced a certain length : such seems to be the ca-. w it h tin-delta 

 of the Nile, which does not advance with the rapidity that mi-lit IK- 

 expected from the quantity of matter brought down by the river. 

 [Nil,K, in Gnoo. Drv.j 



Great as is the amount of new land thus formed, it is insignificant 

 in comparison with the quantity of solid matter carried down by rivers, 

 and deposited in the depths of the sea. It is impossible to form any 

 estimate of this upon which reliance can be placed, because no accurate 

 observations have been mode to supply the data. To come to anything 

 like a satisfactory conclusion, it would be necessary to have a i 

 section of the river at a given point, obtained by numerous H.MII 

 so as to get the profile of the bed, and by observations at .! 

 seasons to get the mean height; we must also have the result - of expe- 

 riments throughout the year, to ascertain the mean velocity, and the 

 volume of solid matter contained in a given bulk of th. water. The 

 quantity of mud and sand ]xmred by the Ganges into the Bay of 

 llengul is so great, in the flood-season, that the sea recovc 

 parency only at the distance of 60 miles from the coast. Sir < 'harles 

 Lyell, in his ' Principles of Geology,' makes a calculation ( founded ujion 

 the computations of Major Kennell) as to the mean quantity of water 

 discharged by the Ganges into the sea, by which he show- th.it, sup- 

 posing the water to contain one hundredth part of solid matter, a mass 

 equal in hulk to the greatest of the Pyramidsof Egypt is brought down 

 by the Ganges every day. The sea is discoloured for many leagues 

 from the mouths of the Orinoco, and the solid contents, swept hy 

 eurrents through the Gulf of Paria, after being partly deposited on tin- 

 shores of (iuiana and the island of Trinidad, are carried into the Carib- 

 bean Sea and Culf of Mexico. By the observations of ( '. .lotn-l Sahine, 

 it appears that the muddy waters of the river Amazonas may be dist in- 

 guished 800 miles from its mouth. The great basin of the Anioy.onaa, 

 which is drained by that mightiest of rivci-s and its vast an.l ooo 

 tributaries, embraces an area, according to Hmnboldt, only on. 

 lem than the whole of Kurope, and through this the main stream (lows 

 for nearly :1IOO miles. The river, at the point win-re its water* unite 

 ith those of the Atlantic, is, according to the same illustrious traveller. 

 In miles bnnd. 



If n river loaded with sand encounter ft marine current at its mouth, 

 the effect frequently is to throw tip a great sand-bank or IBM-, often to 

 tin- detriment of the navigation in the adjoining sea, aii.l 

 to the entire destruction of a harbour. If such sand-hanks be thrown 



