ed by this
streamlet to still greater distances in one day.[272]

_Floods caused by landslips_, 1826.--The power which running water may
exert in the lapse of ages, in widening and deepening a valley, does not
so much depend on the volume and velocity of the stream usually flowing
in it, as on the number and magnitude of the obstructions which have, at
different periods, opposed its free passage. If a torrent, however
small, be effectually dammed up, the size of the valley above the
barrier, and its declivity below, and not the dimensions of the torrent,
will determine the violence of the dabacle. The most universal source of
local deluges, are landslips, slides, or avalanches, as they are
sometimes called, when great masses of rock and soil, or sometimes ice
and snow, are precipitated into the bed of a river, the boundary cliffs
of which have been thrown down by the shock of an earthquake, or
undermined by springs or other causes. Volumes might be filled with the
enumeration of instances on record of these terrific catastrophes; I
shall therefore select a few examples of recent occurrence, the facts of
which are well authenticated.

Two dry seasons in the White Mountains, in New Hampshire (United
States), were followed by heavy rains on the 28th August, 1826, when
from the steep and lofty declivities which rise abruptly on both sides
of the river Saco, innumerable rocks and stones, many of sufficient size
to fill a common apartment, were detached, and in their descent swept
down before them, in one promiscuous and frightful ruin, forests,
shrubs, and the earth which sustained them. Although there are numerous
indications on the steep sides of these hills of former slides of the
same kind, yet no tradition had been handed down of any similar
catastrophe within the memory of man, and the growth of the forest on
the very spots now devastated, clearly showed that for a long interval
nothing similar had occurred. One of these moving masses was afterwards
found to have slid three miles, with an average breadth of a quarter of
a mile. The natural excavations commenced generally in a trench a few
yards in depth and a few rods in width, and descended the mountains,
widening and deepening till they became vast chasms. At the base of
these hollow ravines was seen a confused mass of ruins, consisting of
transported earth, gravel, rocks, and trees. Forests of spruce-fir and
hemlock, a kind of fir somewhat resembling our yew in fol