DESTRUCTIVE ACTION OF TORRENTS. 945 
rapid flow of the surface-waters occasioned no other evil than 
to produce, once in ten years upon the average, an inundation 
which should destroy the harvest of the low grounds along the 
rivers, the damage would be too inconsiderable, and of too 
transitory a character, to warrant the inconveniences and the 
expense involved in the measures which the most competent 
judges in many parts of Europe believe the respective govern- 
ments ought to take to obviate it. 
Destructive Action of Torrents. 
But the great, the irreparable, the appalling mischiefs which 
have already resulted, and which threaten to ensue on a still 
more extensive scale hereafter, from too rapid superficial drain- 
age, are of a properly geographical, we may almost say geologi- 
cal, character, and consist primarily in erosion, displacement, and 
transportation of the superticial strata, vegetable and mineral 
of the integuments, so to speak, with which nature has clothed 
error as the opposite one of ascribing all the agricultural wealth of Egypt to 
that single cause of productiveness. Fine soils deposited by water are almost 
uniformly rich in all climates; those brought down by rivers, carried out into 
salt-water, and then returned again by the tide, seem to be more permanently 
fertile than any others. The polders of the Netherland coast are of this char- 
acter, and the meadows in Lincolnshire, which have been covered with slime 
by warping, as it is called, or admitting water over them at high tide, are 
remarkably productive. 
Recent analysis is said to have detected in the water of the Nile a quantity 
of organic matter—derived mainly, no doubt, from the decayed vegetation it 
bears down from its tropical course—sufiiciently large to furnish an impor- 
tant supply of fertilizing ingredients to the soil. 
It is computed that the Durance—a river fed chiefly by torrents, of great 
erosive power—carries down annually solid material enough to cover 272,000 
acres of soil with a deposit of two-fifths of an inch in thickness, and that this 
deposit contains, in the combination most favorable to vegetation, more azote 
than 110,009 tons of guano, and more carbon than 121,000 acres of woodland 
would assimilate in a year. HEuiste Recius, La Terre, vol. i., p. 467. On 
the chemical composition, quantity, and value of the solid matter transported 
by river, see HERVE MANGon, Sur? Himploi des Laux dans les Irrigations, 8vo. 
Paris, 1869, pp. 182 ef s-qgg. DuroNncuHEL, Traité @ Hydraulique et de Géo- 
logie Agricoles, Paris, 1868, chap, i., xii., and xiii. 
