400 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
much the same.’ Sir William Conway has given an account of a mud-avalanche,’ 
a swift torrent of mud, water, and great rocks, in the Himalayas, somewhat similar in 
nature to these earth-flows. Streams and torrents of mud somewhat analogous but 
usually of glacial or lacustrine origin have been known to flow in the Alps. Mention of 
these has been made by T. G. Bonney.’ 
Earth-flows are important as giving rise to new drainage lines and modifying old ones. 
They are also powerful transporting agents. The initiation of a new drainage line is 
a matter of importance. Once started, it is a point of vantage for the attacks of agents 
of erosion, which thereupon are able to increase their work at an accelerating rate of 
speed. Only next in importance is the definition and fixing of embryonic depressions 
and gullies. Both these processes are carried out vigorously by these earth-flows, 
besides other processes such as the enlargement of valleys and channels already formed, 
the transportation of material, the destruction of the regularity of contours, and the 
transformation of surface rock. material into a form easily removable otherwise, thus 
in every case supplying better leverage for further destructive action. 
Earth-flows usually originate in minor depressions or in already well-formed gullies or 
valleys, these being the places most subject. to the concentration of water: but in some 
instances they occur on the convex face of a slope, where the removal of soil develops 
a depression for the first time, and a new drainage line is made possible. The soft 
débris that is removed, although piled higher than the surrounding slope, lends itself 
easily to the formation of rivulets by the water that rises and collects in the excavation 
that is left. These small water-courses, once formed, control the line of flowage, and 
result in a sort of superimposed drainage when they have worn through the débris to 
the original slope below. Earth-flows of the above varieties, large and small, with the 
closely related types of ear th-slumps, are thus among the important initial steps in the 
development of drainage lines in the California hills. 
EARTH-LURCHES. 
Of the three kinds of Jandslides thus far referred to, the first two, earth-avalanches 
and earth-slumps, occur quite commonly independent of earthquakes. Of the third 
kind, or earth-flows, the only examples that have been presented are immediately con- 
nected in genesis with the earthquake of April 18, although it is conceded that sudden 
accessions of water to loose earth might arise in other ways and occasion earth-flows. 
As regards the fourth type, the earth-lurch, it is difficult to conceive for it any other 
origin than an earthquake, since it is caused directly by the horizontal jerk of the ground 
and can not be produced in any other way. In the detailed account of the distribution 
of apparent intensity, a brief account of these superficial movements of the ground has 
been given and need not here be repeated. They are best exemplified on the flood 
plain of the Eel River, west and north of Ferndale; the flood plain of the Russian River ; 
the flood plain of Alameda Creek, near Alvarado; the flood plain of Coyote River near 
Milpitas; the flood plain of Pajaro River; and the flood plain of the Salinas River. 
(Plates 136a,B and 137a,8B.) In all these. Tees cracks were formed in the alluvium, 
generally parallel to the stream trench, and the ground between the cracks was caused 
to lurch horizontally toward the stream, usually with a rotation of the moved mass, 
which gave to it the profile of a Basin Range fault-block in miniature, the portion of 
the moved strip farther from the stream collapsing into the vacuity caused by the 
lurching. 

1G. A. J. Cole, Nature, Jan. 14, 1897, vol. 55, pp. 254-256. G. H. Kinahan, Nature, Jan. 21, 1897, 
vol. 55, pp. 268-269. 
2 W. M. Conway, Climbing in the Himalayas. New York, 1894, pp. 118, 129-130, 323-324. 
’'T. G. Bonney, Moraines and Mud Streams in the Alps. Geol. Mag., January, 1902, p. 8. 
