SEDIMENT BROUGHT DOWN BY RIVERS. 159 



The substances transported by the stream, and deposited along 

 its sides, are, of course, such as are furnished by the hills around its 

 sources, and above its channel ; and the almost incessant accumula- 

 tions of earthy matter which thus take place, may be varied, according 

 to the nature of the country, by interposed layers of vegetable re- 

 mains. In tropical and warm regions, and in unenclosed countries, 

 this is the case to a far greater extent than an acquaintance with Euro- 

 pean rivers would lead us to expect. The mighty forests of America, 

 untouched by human industry, annually furnish to the great rivers 

 which intersect them an immense spoil of trees, which being easily 

 supported by the current, are carried towards the sea, and deposited 

 at the river mouth, or drifted away on the waves. 



Arrangement of Materials. The arrangement of the materials 

 brought down by streams is in general regulated by a tendency to the 

 production of a level surface, and thus the original inequalities of a 

 valley are continually lessened. In a high region like the Alps, the 

 rough streams leave in the higher levels chiefly a collection of pebbles 

 and sand in local confusion ; but the general effect is a uniformly 

 declining plane, through which the capricious stream finds for itself 

 new channels, and thus continually shifts its deposits over the whole 

 broad pebbly floor of the river valley. Such effects may be well 

 seen on the line of the Arve, as it hurries down from the glaciers of 

 Savoy. On the contrary, in the lower and more level expansions of a 

 valley, where the gentler waters transport only fine sediment and 

 vegetable substances, these materials are arranged in most exact 

 parallelism over a large extent of plane surface, and by counting the 

 laminae of deposition, some notion may be formed of the period occu- 

 pied in the process. On the borders of streams which are periodically 

 swollen by rain, as in the tropical regions, or by the melting of snows, 

 as in those which descend from high mountain countries, this mode of 

 computation of the laminae may even be trusted so far as to determine 

 the number of years employed in producing a given depth of deposit ; 

 and in districts where the rivers swell irregularly at uncertain in- 

 tervals, there might be deduced an average rule as to the rate of 

 deposition. Nor would the accumulation during a short period of 

 time, tried by this test, appear inconsiderable. In a single season, the 

 rivers of Yorkshire, aided by the sea, deposit many inches of rich soil 

 upon the level peat-moors which adjoin their estuary ; and at Ferry- 

 bridge, at the point where the tide, formerly flowing up the river, 

 neutralised the freshes of that river, many of the modern works of 

 man, as oars of a boat, a coin of England, were found buried under 

 the alluvial sediment, which contained petrified hazel branches and 

 nuts, bones of deer, &c. The rivers of the Bedford Level have con- 

 stantly silted up in historic times. The Great Ouse formerly (1292) 

 joined the Nene and flowed out at 'Wisbeach, but as that outfall 

 became silted up owing to the deposit of sediment by the stagnating 

 waters, the whole drainage of the Level was diverted towards the 

 outfall at King's Lynn. The inundations of the Nile raise the land 

 of Egypt 4^ inches in a century. 



