THE EARTHQUAKE AS A NATURAL PHENOMENON. 13 
made where the formations are alluvial. Doubtless many of the 
observations are fallacious, depending on a subjective illusion as to 
the direction of verticalit} 7 , which arises from the horizontal move- 
ments of the observer's support ; but some of them may also be 
objective. Not only is there a gradation in physical condition from 
dry earth through mud to water, but the shaking of a loose formation, 
whether wet or dry, overcomes the adhesion of particles and thereby 
imparts for the time being a mobility analogous to that of liquids. 
It is therefore conceivable that gravity waves, altogether analogous to 
those of water, may be produced by a violent earthquake on the sur- 
face of a loose formation. Certain ridges on soft ground caused by 
earthquakes in Japan are inferred by Omori and Kikuchi to repre- 
sent such soil waves and to indicate a wave length (crest to crest) of 
20 to 40 meters. The San Francisco earthquake produced a similar 
ridging on tidal mud in Tomales Bay, the average ridge interval 
being not more than half that of the Japanese examples. The 
Tomales Bay ridges are roughly parallel to the fault trace (which is 
close at hand), have about the same irregularity as wind waves, and 
originally ranged in height from 1 to 3 feet. They were observed 
chiefly, but not exclusively, on the body of mud already mentioned as 
having been shifted toward the southwest shore. The tract shown in 
PI. VII lies so low as to be submerged much of the time, and the 
ridges had been so nearly obliterated when the views were taken that 
little remained besides an obscure indication of their ground plan; 
but an area nearer the head of the bay, and probably lying somewhat 
higher, not only seemed to have preserved its character when photo- 
graphed (April 28; see PL VIII, A), but showed little change when 
visited nine months later. 
Notwithstanding the persistence of the ridges at the last-mentioned 
locality, there is no reason to question the statement that the whole 
mud plain had the smooth surface common to tidal flats until it was 
disturbed by the earthquake. Nor do I find any room for doubt 
either that the ridges originated as waves on the surface of the mud 
while it was rendered quasi liquid by violent agitation, or that they 
persisted because the mud promptly resumed its normal coherence 
when the agitation ceased. It is by no means equally clear that the 
arrested waves were true gravity waves rolling across the mud plain. 
Whatever their mechanism and history, they illustrate a mode of 
response of wet, unconsolidated material to powerful earth tremors, 
they suggest an explanation of certain wave] ike ridges produced on 
areas of made ground in San Francisco, and they contribute to an 
understanding of the peculiar destructiveness of the earthquake in 
such areas. 
