262 SCOOPING OUT OF RIVER-BEDS. 
bridge near La Beaume, which had been built about eighty years 
before. The resistance of the piers, which were erected on piles, 
the channel at that point being of gravel, produced an eddying 
current that washed away the bed of the river above them, and 
the foundation, thus deprived of lateral support, yielded to the 
weight of the bridge, and the piles and piers fell up-stream. 
By a curious law of compensation, the stream which, at flood, 
scoops out cavities in its bed, often fills them up again as soon as 
the diminished velocity of the current allows it to let fall the 
sand and gravel with which it is charged, so that when the 
waters return to their usual channel, the bottom shows no sign 
of having been disturbed. In a flood of the Escontay, a tribu- 
tary of the Rhone, in 1846, piles driven sixteen feet into its 
gravelly bed for the foundation of a pier were torn up and e¢ar- 
ried off, and yet, when the river had fallen to low-water mark, the 
bottom at that point appeared to have been raised higher than 
it was before the flood, by new deposits of sand and gravel, 
while the cut stones of the half-built pier were found buried to 
a creat depth in the excavation which the water had first washed 
out. The gravel with which rivers thus restore the level of their 
beds is principally derived from the crushing of the rocks brought 
down by the mountain torrents, and the destructive effects of 
inundations are immensely diminished by this reduction of large 
stones to minute fragments. If the blocks hurled down from 
the cliffs were transported unbroken to the channels of large 
rivers, the mechanical force of their movement would be irresist- 
ible. They would overthrow the strongest barriers, spread 
themselves over a surface as wide as the flow of the waters, 
and convert the most smiling valleys into scenes of the wildest 
desolation. 
As I have before remarked, I have taken my illustrations of 
the action of torrents and mountain streams principally from 
French authorities, because the facts recorded by them are 
chiefly of recent occurrence, and as they have been collected 
with much care and described with great fulness of detail, the 
information furnished by them is not only more trustworthy, but 
