340 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
has been deprest: below the local base-level. This may be taken as an indication of the 
persistence of the compressive forces which originally gave rise to the syncline. If, now 
the underlying rocks of the valley were in a state of synclinal stress, the relief of that stress 
afforded by the dislocation along the line of the Rift might give rise to an elastic dis- 
turbance of the ground which would be additive to the shock generated at the fault along 
the Rift. 
But none of these suggestions, whether of contributory shock, or an unrevealed fault, 
or of coincidence of the valley with a vaguely defined zone of maximum horizontal 
jerk, or of sudden relief from synclinal compression, are sustained by satisfactory evidence. 
They are possibilities which, with the facts before us, it is possible neither to affirm nor 
to deny. The reference to them in this place is only excusable on the ground that they 
are suggestive of lines of inquiry which may perhaps be profitably undertaken at some 
future time. On the other hand, the influence of the character of the ground upon the 
apparent intensity is sustained by cumulative evidence. 
In Sonoma and Napa Valleys, the disposition of the isoseismals is very evidently de- 
termined by the contour of the valleys, the high intensities running far up the valleys 
within areas of lower intensity on either side. In Sonoma Valley the upper and lower 
parts are alluviated, while the middle part is not; or, if so, only to a slight extent, and it 
is being trenched by the stream which flows thru it. The floor of Napa Valley, on the 
other hand, is alluviated thruout, save for some rocky spurs and isolated rocky hills which 
occur along portions of the sides of the valley. The intensity diminishes in the upper part 
of Napa Valley, in the vicinity of Calistoga, where the alluvial deposits thin out, notwith- 
standing the fact that Calistoga is somewhat nearer the fault along the Rift than is 
Napa City, at the lower end of the valley, and notwithstanding the fact that Calistoga is 
approximately on the line of the Mount St. Helena fault described by Osmont. If the 
relatively high apparent intensity of Napa Valley were in any way referable to a local 
earthquake on a fault traversing the valley, we should not only expect the effects to be 
manifested on the rocky slopes of the valley, as well as upon its floor, but would also expect 
higher intensities on the line of the only well-defined fault known to traverse the valley. 
Neither of these expectations is realized, and upon the slopes of Mount St. Helena, in the 
vicinity of the fault which traverses its western front, the intensity was notably low —not 
higher than VI. We are thus again forced to fall back upon the character of the ground 
as the immediate cause of the high apparent intensity on the alluviated valley-floor, 
particularly in the lower part of the valley. 
Specific and instructive instances of the influence of the character of the ground upon 
the apparent intensity of the shock are afforded by the cities of Petaluma and San Rafael. 
Each of these cities is built partly upon rock and partly upon the alluvium of the tidal 
marshes of the San Francisco Bay. Petaluma is situated at a distance of 14 miles from 
the fault, and San Rafael at a distance of 9 miles. In both cities the damage to buildings, 
chimneys, etc., was notably less upon the rock than upon the alluvium, altho the latter 
can not in either case be supposed to have any great thickness at the base of the hills. 
(See fig. 64.) 
In the city of San Francisco the detailed study of the distribution of intensity, so suc- 
cessfully carried out by Mr. H. O. Wood, affords a conclusive proof of the paramount 
influence of the character of the ground in determining the high apparent intensities 
which affected portions of the city. On the made land in the vicinity of the Ferry 
Building, about 9.5 miles from the fault, as well as on the tidal marsh jand, and along 
Mission Creek and Lagoon, between 7 and 9 miles from the fault, the intensity was X of 
the Rossi-Forel scale. But on the rocky top of Telegraph Hill, near the ferries, the in- 
tensity was scarcely higher than VII. On the sandstone cliffs at Point Lobos, about 3 
miles from the fault, it was about VIII; and on the summits of the chert hills in the cen- 
