ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 339 
northward to Mark West Creek, the drainage is stagnant and gives rise to the Laguna de 
Santa Rosa. This lagoon is a drowned water course in free connection with the trunk 
drainage of the Russian River, and is indicative of a deformation of the valley surface 
whereby the western side has been deprest below the base-level established by the Russian 
River. From these statements it will be apparent that the whole of the floor of the Santa 
Rosa Valley is not alluviated, but that portions of it — particularly that portion lying 
between Mark West Creek and Healdsburg and east of the flood plain of the Russian 
River — is a terraced platform carved out of the Merced terrane. 
Now the notably high apparent intensity of the earthquake shock was confined to the 
alluviated portion of the valley-floor. The 2 centers of population which suffered most 
severely were Santa Rosa and Sebastopol. At Windsor, situated on the terrace cut in 
the Merced rocks, the intensity was distinctly lower. Healdsburg, at the northern extrem- 
ity of the valley, is also on alluvium and the intensity was here again high, tho not quite 
equal to that at Santa Rosa and Sebastopol. The town of Guerneville, on the old flood 
plain of the Russian River, below the Santa Rosa Valley, suffered most severely; while 
the cemetery of the town, but a short distance away, on a rocky terrace 190 feet above 
the town, was affected in a distinctly less degree, only one monument having been over- 
thrown, and a few moved on their pedestals. The rapid diminution of intensity on pass- 
ing from the alluvium to the rocky slopes, thus specifically illustrated at Guerneville, is 
characteristic of the borders of the Santa Rosa Valley. To the east of the city of Santa 
Rosa, this diminution is so rapid that the gradation of intensity can not be adequately 
exprest upon the intensity map. Under these circumstances it is difficult to avoid the 
conclusion that the severity of the earthquake shock on the alluvium of the Santa Rosa 
Valley is in large measure referable to the character of the ground. Were a local shock 
a factor in the case, we should expect that the high intensity would not be limited to the 
alluviated area, but would also be manifested on the surrounding mountain slopes. This 
expectation not being realized, the hypothesis of an independent local shock stands with- 
out support. ‘The general position of the isoseismal curves off the valley-bottoms is not 
notably affected by the high apparent intensity in the valleys. In arriving at the con- 
clusion that the high apparent intensity in this valley is referable in large measure to the 
character of the ground, it is not thereby intended to exclude other contributory factors. 
A theoretical discussion of the effect at the surface of the earth of a concussion at a point 
within the crust shows that for a certain path of emergence the horizontal jerk of the 
emerging earth-wave, and, therefore, the destructive effect in general, would be at a max- 
imum. The fact that the earthquake under consideration was due not to a concussion at 
a point, but to a jar developed by movement on a plane at least 270 miles long, reaching 
to the surface and of unknown depth, renders the application of this doctrine difficult 
and of questionable value. Nevertheless, the tendency, which is demonstrable in the 
ideal case, would also exist in the more complex actuality; and it is by no means impos- 
sible that the zone of maximum destruction may fall in a general way within the Santa 
Rosa Valley, and would thus be a factor conducive to excessive destruction, in addition 
to the factor inherent in the character of the ground. This suggestion, to have weight, 
should be corroborated by observations in other portions of the general zone of destructive 
effects, and it must be confest that satisfactory corroboration is lacking. 
While the geology of the Santa Rosa Valley has not been mapped in detail, owing to the 
lack of topographic maps, it has been carefully studied, particularly from the structural 
and stratigraphic point of view, by Mr. Vance Osmont,! and no fault traversing the valley 
was found by him. The underlying structure, so far as has been made out, is as already 
stated that of a broad, rather simple, synclinal fold. It has also been indicated that the 
surface of the valley has been subjected to recent deformation, whereby the western side 
? Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3. 


