THE EARTHQUAKE AS A NATURAL PHENOMENON. 9 
the vertical change as much as 2 feet. As the ascending movement 
can not be ascribed to gravity, it must be referred to the earthquake, 
even though the way in which the earth waves produced the effect is 
not evident. The locality is adjacent to the fault trace, the position 
of which is along the bottom of the bay, east of the shoal. 
The illustrations may require a few words of explanation. The 
upper view of PL VII looks northward from the southwest shore 
of the bay. Tide being low, the newly formed shoal or mud bank is 
broadly exposed, but the receding tide has left a lane of water to 
mark the separation of the mud bank from the firmer ground that 
withstood the quaking. Immediately after the earthquake the mud 
was rigid, as in the tract shown in PI. VIII, .1/ but before the view 
of PI. VII, A, was taken (April 28, 190G) the surface had been largely 
smoothed by the action of wind waves. A single ridge which es- 
caped that action appears at the left in the upper view of PL VII 
and in the foreground of the lower view. 
A permanent disturbance of the ground also resulted in many 
instances from compacting. Just as sand or grain that has been 
poured into a measure can be made by shaking to settle down and 
occupy less space, so various loose formations, and especially arti- 
ficial fillings, were shaken tog-ether by the earthquake and the ground 
surface lowered. In such compacting the particles making up the 
aggregate are readjusted so as to fit more closely together and the 
voids are reduced. In dry formations compacted by the earthquake 
the reduction of voids was opposed only by the elasticity of the 
contained air. In wet formations it encountered the effectual resist- 
ance of the contained water, and could be accomplished only by the 
extravasation of some of the water. Ordinarily it was impossible to 
measure the settling due to compacting, or even to determine its 
occurrence as a phenomenon independent of ground flow, but it was 
clearly seen in various localities in San Francisco where those parts 
of graded streets which retained their simple shapes and straight 
lines served as reference planes for neighboring parts that were 
disturbed. (Compare the distance and foreground of PL V. 
Another example of the effect on the filled-in land in this part of 
the city is shown in PL VI, A, a view of Dore street between Bryant 
and Brannan streets. The settling of the soft ground caused the 
street to drop at least 5 feet at this place.) 
The only notable water waves generated by the shock were in 
Tomales Bay, where a group of waves estimated to be 6 or 8 feet high 
came to the northeast shore. The automatic tide gage at the Presidio 
showed a depression of about 4 inches, Avith subsequent oscillations 
of similar amount. Water was spilled from tanks, etc., and in at 
least one place was thrown from a pool out on the land. 
