432 SAND BANKS-SEDIMENT OF THE NILE. 
widening basins, and forms plains of moderate extent; the fine 
silt wliioli floats farther is deposited over a still broader area, 
or, if carried out to sea, is, in great part quickly swept far off 
by marine currents and dropped at last in deep water. Man’s 
u improvement ” of the soil increases the erosion from its sur¬ 
face ; his arrangements for confining the lateral spread of the 
water in floods compel the rivers to transport to their mouths 
the earth derived from that erosion even in their upper course; 
and, consequently, the sediment they deposit at their outlets is 
not only much larger in quantity, but composed of heavier 
materials, which sink more readily to the bottom of the sea 
and are less easily removed by marine currents. 
The tidal movement of the ooean, deep sea currents, and 
the agitation of inland waters by the wind, lift up the sands 
strewn over the bottom by diluvial streams or sent down by 
mountain torrents, and throw them up on dry land, or deposit 
them in sheltered bays and nooks of the coast—for the flowing 
is stronger than the ebbing tide, the affluent than the refluent 
wave. This cause of injury to harbors it is not in man’s 
power to resist by any means at present available ; but, as we 
have seen, something can be done to prevent the degradation 
of high grounds, and to diminish the quantity of earth which 
is annually abstracted from the mountains, from table lands, 
and from river banks, to raise the bottom of the sea. 
This latter cause of harbor obstruction, though an active 
agent, is, nevertheless, in many cases, the less powerful of the 
two. The earth suspended in the lower course of fluviatile 
currents is lighter than sea sand, river water lighter than sea 
water, and hence, if a land stream enters the sea with a con¬ 
siderable volume, its water flows over that of the sea, and 
bears its slime with it until it lets it fall far from shore, or, as 
is more frequently the case, mingles with some marine current 
and transports its sediment to a remote point of deposit. The 
earth borne out of the mouths of the Nile is in part carried 
over the waves which throw up sea sand on the beach, and 
deposited in deep water, in part drifted by the current, which 
sweeps east and north along the coasts of Egypt and Syria, 
