ALLUVIUM. 



ALLUVIUM. 



of its own territory. Those districts which are 

 thus added to the land are usually superposed 

 by a fine rich alluvial soil, as also are those 

 which have at a former period been covered 

 by the sea, and would be at the present day, 

 were it not for the ingenuity and works of man. 



The districts in which are situated New Or- 

 leans in America, and Missolonghi in Greece, 

 are chiefly alluvial, and nearly the whole of 

 Holland has the same character, and can only 

 be described as a district of which man has 

 robbed the ocean. That part of the coast of 

 Germany which is bordered by the North Sea 

 is alluvial, and additions are constantly made 

 to the shores by the gradual depositions of 

 earthy matter upon the immense flats which 

 extend along them. The first sign of vegeta- 

 tion on these lands is the appearance of the 

 saltwort (Salicornia maritima), which is suc- 

 ceeded by the sea grass (Poa maritima}, and 

 when the land is very rich, by the marsh star- 

 wort (Aster Tripolium). The land is after- 

 wards dyked, and used as pasture for sheep 

 and cattle ; so that the spot over which the sea 

 has perhaps for ages exercised an undisputed 

 control, is brought under the power of man in 

 a state most admirably adapted tosuit his wants. 



In Lincolnshire and other parts of the Eng- 

 lish coast, where the land is beneath the level 

 of high-water mark, unfruitful districts are 

 often restored to a state of fertility by the re- 

 moval of the artificial banks that prevent the 

 sea-water from flowing over it. In this 

 way the land is thrown open to the sea, 

 and as the tide rises, it is covered by water, 

 which, being overcharged with earthy matter, 

 deposits in two or three years a bed five or 

 six feet thick of rich soil, which may be 

 brought under cultivation by the exclusion of 

 the agent that was instrumental in its produc- 

 tion. (See WARPING.) 



But it may be asked, whence does the sea 

 obtain the earthy matter with which it abounds ? 

 Rivers discharge themselves into the ocean, 

 and it has been already stated that their waters 

 are charged, more or le^s, with the superficial 

 soil of mountainous countries, and the de- 

 stroyed materials of rocks. A part of this 

 may be arrested by occasional or periodical 

 inundations, and by deposition in the bed of 

 the river, but a large quantity must still be 

 carried into the ocean. It must also be re- 

 membered that the water which is conveyed 

 in a channel is constantly endeavouring so 

 to arrange its coarse as to suffer the least 

 possible resistance. In this attempt, it attacks 

 the banks that confine it, and widens its 

 course, precipitating much earthy matter into 

 the stream, to be removed by the flowing water. 

 It frequently happens, and especially after the 

 fall of heavy rains, that the water at the mouths 

 of rivers is thick and turbid from the quantity 

 of alluvial matter it holds in solution, and very 

 many large rivers are rendered unsafe for na- 

 vvfcation by the existence of large bars of sand 

 or^clay at their outlet. 



But the sea is not merely a passive recipi- 

 ent of the product of destructive causes, but 

 is itself a cause. Sea coasts are constantly 

 suffering depredation by the action of the 

 waves that beat upon them. Whether we look 

 70 



at the soft and almost unresisting rocks of the 

 eastern coast of England, or the hard primary 

 rocks of Devonshire, Cornwall, and the Shet- 

 land Isles, the same results will be observed. 

 During the stormy months of winter, when 

 the waves are tossed upon the coasts with an 

 almost uncontrolled violence, no rock is suffi- 

 ciently hard to resist its energy, arid when un- 

 ruffled by a passing breeze in the months of 

 .summer, its influence upon the softer rocks is 

 hardly less destructive, though more insidious, 

 for it then attacks the base of the cliffs, and 

 removing the support of the superincumbent 

 mass, causes the precipitation of large portions 

 into the sea. By these two causes the sea is 

 provided with the materials for the formation 

 of alluvial soils. Some estimate may be 

 formed of the violence and extent of these 

 causes, by an examination of the present 

 state of the German Ocean, one fifth of which 

 is covered by banks that appear to have been 

 produced in the same way as the alluvial soils 

 on the northern coast of Germany. 



Water, then, is a most powerful agent in the 

 destruction and production of rocks, and were 

 there no conservative principle, the changes 

 that are going on would be more extensive 

 than they are in the present day. The floods 

 to which some rivers are subject are so impe- 

 tuous that they frequently sweep away all op- 

 posing objects, and involve an entire district 

 in ruin. These effects, however, are much 

 more common in countries that are thinly 

 covered by vegetation than in those where it 

 is luxuriant, for it acts as a conservative agent, 

 increasing the power of the resistance, by 

 binding the soil more closely together. This, 

 therefore, will account for the diminished influ- 

 ence of floods upon lowlands, and for the fre- 

 quent deposition of rich and fertile alluvial 

 soils. 



The composition of the alluvial soils that 

 have been brought under cultivation is exceed- 

 ingly various ; but they are generally re- 

 markable for their fertility, and are admirably 

 suited for pasture lands. "In general," says 

 Sir Humphry Davy, "the soils, the materials 

 of which are most various and heterogeneous, 

 are those called alluvial, or which have been 

 formed by the deposition of rivers ; many of 

 them are extremely fertile. I have examined 

 some productive alluvial soils, which have 

 been very different in their composition. A 

 specimen from the banks of the river Parret in 

 Somersetshire, afforded me eighty parts of 

 finely divided matter, and one part of silicious 

 sand ; and an analysis of the former gave the 

 following result ; 



Carbonate oflime - - - - - 360 parts. 

 Alumina -------25 



Silica 20 



Oxide of iron ------ 8 



Vegetable, animal, and saline matter - - 19 



" A rich soil from the neighbourhood of the 

 Avon, in the valley of Evesham, in the Wor- 

 cestershire, afforded me three-fifths of fine sand 

 and two-fifths of impalpable matter. This last 

 consisted of 

 Alumina -------41 parts. 



Silica 42 



Carbonate oflime ----- 4 

 Oxide of iron --__--; 

 Vegetable, animal, and saline matter - 8 



