144 ELEMENTARY GEOLOGY 
passage of water through some of the porous materials 
to more impervious layers; as, for imstance, when 
water passes through sand beds to underlying rock, or 
a layer of clay. ‘Tricklmg along the surface of this 
impervious mass, the water lubricates it and causes a 
slipping plane, over which, if ready, the upper mass 
of earth may slide. During every rain there is some 
motion of this nature among the surface particles of 
soil; and on many hillsides we may find little scars 
formed where masses of earth have slipped so recently 
that vegetation has not grown over the place from 
which the earth has moved, 
Upon steep mountain sides, a landslide or avalanche 
may start high up near the top, and rush downward 
with terrific violence, gathering force and size as it 
descends, and ploughing a path of destruction even 
through a dense forest. As it falls, it causes a violent 
wind which is sometimes strong enough to overturn 
houses. Landslips occur, too, where streams or ocean 
waves are undermining cliffs (Fig. 71). 
Noticeable though they are, landslips really accom- 
plish less than slower actions which we do not com- 
monly notice; for these are constantly at work all over 
the earth, while the landslide, though violent, is local 
and comparatively rare. Of more widespread impor- 
tance than avalanches, is the minute sliding of the soil 
particles which are rendered slippery during rains 
