386 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
would inevitably be the enlargement of the area of the slide in the wet season. Sim- 
ilarly on many slopes, particularly at points not far distant from the Rift, numerous 
cracks were opened without actual slumping of the ground occurring in consequence of 
the shock; but the conditions were thus provided for the slumping process the following 
winter. During the winter 1906-1907 many such slides were reported in a general way. 
Unfortunately detailed information as to their occurrence is as yet lacking. It is to be 
noted that an exceptionally heavy rainfall conspired with the conditions established 
by the earthquake to produce these. landslides. 
In the type of landslide thus far considered, the contained water, which is at once in 
part the cause and the means of the movement, accumulates relatively slowly, and it 
varies with the season, there being usually a more or less free drainage from the lower 
portion of such slides. There are, however, other landslides which are due to a relatively 
large and sudden accession of water to the unconsolidated materials of a slope. Such 
sudden accessions of water may be conceived to be produced in a variety of ways; such, 
for example, as a so-called “cloudburst” in a desert canyon, the slopes of which may 
be heavily mantled by earth and loose rock; or the breaking of a barrier which retains 
a bog or other body of water. For the present purpose, however, which is not that of an 
exhaustive systematic discussion of this class of phenomena, it will be sufficient to take 
note only of water which is expelled from the ground by the compressive action of the 
earthquake shock. Such landslides may be discriminated from earth-slumps by reason 
of their greater mobility, under the designation earth-flow. Earth-flows differ from 
sarth-slumps not only in the much larger quantity of water involved in their mechanism 
as a moving mass, in the suddenness with which the water becomes efficient as a trans- 
porting agency, and in the rapidity of the movement; but also in the brevity of the 
entire process, its finality, and its non-recurrence. 
Besides these two types of landslides, there is still another, which is immediately asso- 
ciated with earthquakes as a cauise of movement. ‘This is the slide of dry earth and 
rock upon precipitous slopes or their fall from cliffs. Soil or other loose forms of earth 
may participate in such landslides, but the material is usually composed chiefly of rock 
which becomes increasingly shattered with the progress of the slide. Such landslides 
will here be referred to as earth-avalanches. They are distinguished from both earth- 
slumps and earth-flows by the character of the material and by the absence of water 
as gn essential factor in producing movement. They also differ usually in the marked 
acclivity of the slopes on which they occur. They differ from earth-slumps, but resemble 
earth-flows, in the finality or completeness of the movement. They are not progressive 
movements, but sudden events; and there is no recurrence of movement of the material 
involved, altho the avalanche may recur at the same place. 
Besides these three types of landslide, another ought perhaps to be recognized. This 
is the form of superficial earth movement which occurred in consequence of the earth- 
quake shock on the alluvial bottom-lands of many streams. It may appropriately be 
designated an earth-lurch. It varies from the opening of a mere crack, with a slight 
movement of the ground on one or both sides, to a violent and complicated deformation 
of the surface, usually accompanied by cracks and open fissures parallel to the trend 
of the neighboring stream trench. These cracks and fissures cut the ground up into 
strips or prisms which lurch toward the stream trench, or, it may be, toward an aban- 
doned slough, the lurch usually being accompanied by a rotation of the prism. They 
are distinguished from all other forms of landslides by occurring on perfectly flat. ground 
and by the fact that they are apparently referable directly and solely to the horizontal 
jerk of the earth movement during the earthquake shock. 
A brief account, which in some cases amounts only to a mention, will now be given 
of some of the various kinds of landslides set in motion by the earthquake. 
