long rains and after winter frosts the cohesion between the waste
and the sound rock beneath is loosened by seeping water
underground. The waste slips on the rock surface thus lubricated
and plunges down the mountain side in a swift roaring torrent of
mud and stones.

We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved
in the sudden movement. Such slips occur when valleys have been
rapidly deepened by streams or glaciers and their sides have not
yet been graded. A favorable condition is where the strata dip
(i.e. incline downwards) towards the valley (Fig. 11), or are
broken by joint planes dipping in the same direction. The upper
layers, including perhaps the entire mountain side, have been cut
across by the valley trench and are left supported only on the
inclined surface of the underlying rocks. Water may percolate
underground along this surface and loosen the cohesion between the
upper and the underlying strata by converting the upper surface of
a shale to soft wet clay, by dissolving layers of a limestone, or
by removing the cement of a sandstone and converting it into loose
sand. When the inclined surface is thus lubricated the overlying
masses may be launched into the valley below. The solid rocks are
broken and crushed in sliding and converted into waste consisting,
like that of talus, of angular unsorted fragments, blocks of all
sizes being mingled pellmell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.

At Gohna, India, in 1893, the face of a spur four thousand feet
high, of the lower ranges of the Himalayas, slipped into the gorge