MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE, 

LANDSLIDES. 
There are three types of landslides known in the Coast Ranges. For convenience 
in reference they may be designated as earth-avalanches, earth-slumps, and earth- 
flows. The first and last of these are of somewhat. exceptional occurrence, but the 
second is exceedingly common. These landslides are of geological importance as an 
agency concerned in the evolution of the geomorphy of the Coast Ranges of California 
to an extent equaled in few other regions; and it becomes a matter of interest to appre- 
ciate the rdle played by earthquakes in promoting the efficiency of this agency. The 
activity of all three kinds of landslides is related directly or indirectly to the earthquake 
of April 18, 1906. In order to appreciate certain phases of the relationship, it will be 
of advantage to state briefly, in a general way, some of the characteristics of these dif- 
ferent types of landslides. In doing this, reference will first be made to the most com- 
monly occurring type, the earth-slump. The other two may then be characterized by 
contrast with this type. 
Under normal conditions, earth-slumps appear chiefly as features of mature slopes 
which are in adjustment to the ordinary processes of rain erosion. They are also found, 
however, as notable features of immature slopes, at the base of which horizontal cor- 
rasion is active, as on sea-cliffs and stream-cliffs, supplanting under certain conditions 
the earth-avalanche which is chiefly found in such situations. On the mature slopes of 
the Coast Ranges of California, under present climatic conditions, the regolith or mantle 
of decomposed rock, on the more common rocks, appears to be accumulating at a some- 
what faster rate than the rain-wash can remove it. This excessive accumulation of the 
regolith appears to be an important factor in producing conditions conducive to earth- 
slumps. The climate of the region is characterized by a pronounced alternation of 
dry and wet seasons. In the summer the soil and regolith on the hillsides are dried out 
to a considerable depth, in many cases down to the underlying firm rock; and as the 
desiccation proceeds the soil shrinks and cracks. The cracks thus formed permit the 
ready access of the early winter rains to the deeper portions of the soil and regolith. The 
concentration of the entire rainfall in one half of the year is also more conducive to the 
saturation of the ground than if it were distributed thruout the year. The climate 
is thus a contributory factor to the prevalence of earth-slumps. 
A factor of local importance is the character of the underlying geological formations. 
Where these consist of clays or shales, earth-slumps are much more liable to be inau- 
gurated and to recur than where the rocks have little or no clay in them. The emergence 
of springs on hillsides is also a fruitful cause of earth-slumping where other conditions, 
particularly the last mentioned, are favorable. Another factor may be the recent sub- 
jection of the hill-slopes to grazing and tillage. In general, however, this interference 
with natural conditions appears to have been conducive to excessive corrasion and 
sapping, rather than to slumping. Grazing and tillage rob the surface of its natural 
protection of dead grass and other vegetation, which in the early winter season tend to 
restrain the rapid flow of the rain-water and its concentration in lines of corrasive activity. 
New lines of corrasion are thus inaugurated, and where the rocks are but slightly coherent 
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