8 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
the earth on the morning of April 18, 1906, supplied the lacking 
factor, and they were all loosened at once. In the simplest case, a 
poised rock toppled over and rolled down a slope. In other cases 
adhesion was overcome, with resultant sliding. In yet others strains 
occasioned by the sapping of cliffs were reinforced by kinetic strains 
and cohesion was overcome, with resultant fracture. Elsewhere an 
unconsolidated formation, even though in a dry condition, was made 
to flow by simple agitation. Hillside bogs and other bodies of wet 
earth lost coherence and flowed as mud. Slips of this character grade 
into those of wet alluvium or " made ground " resting upon gentle 
slopes — ground which under ordinary conditions flows or creeps at an 
almost imperceptible rate, -but which by shaking was made to move 
several feet or yards in a few seconds. The filled districts of San 
Francisco afford several examples, and two of these are illustrated by 
Pis. V and VI, B. The view shown in PI. V is northwestward on 
Ninth street, near Brannan. Before the earthquake the car tracks 
and curb line were straight and approximately level, and this condi- 
tion was not disturbed on the relatively firm ground shown in the 
distance. In the nearer part of the view the street crosses a tract of 
made ground created by filling a valley tributary to a narrow tidal 
inlet called Mission Creek. The descent of this valley was south- 
westward, and the made ground flowed in that direction, carrying 
street and buildings with it. In taking the photograph reproduced 
in PI. VI, B, the camera stood on ground made by the filling of Mis- 
sion Lagoon, an expansion of Mission Creek, and was pointed north- 
ward, commanding a portion of Howard street. The made ground 
here flowed northeastAvard and the buckling of street-car tracks was 
caused by its motion. Where the same earth flow crossed Valencia 
street the horizontal movement amounted to 6 feet. 
In the various cases of dislocation enumerated the motive force 
appeared to be gravity, and the apparent function of the earthquake 
was to initiate movement by overcoming equilibrium, adhesion, or 
cohesion, or else to increase mobility by agitation, and thereby tem- 
porarily convert a quasi solid into a quasi liquid. While I do not 
doubt that this explanation is ordinarily adequate, there is at least 
one dislocation of surface material for which it is inadequate, and 
this raises the question whether in various other instances it may 
not require qualification. I refer to an extensive shifting of mud on 
the bottom of Tomales Bay. At the head of the bay and thence for 
a distance of several miles northwestward the soft mud was moved 
bodily westward. It not only descended from the northeast shore, so 
as to cause deeper water, but ascended toward the southwest shore, 
creating a broad shoal (PI. VII). The horizontal change of posi- 
tion near the southwest shore was in places more than 25 feet, and 
