ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 197 
Distribution. — The variation of intensity with the character of the geologic formation 
is evident at various localities, but most conspicuously at Bolinas, where the destruction 
on alluvium at the bottom of the little valley was very much greater than on the hills 
immediately adjacent. Nevertheless, the data are not sufficiently full for a satisfactory 
discussion of this phase of the distribution of intensity, and I have therefore tried fo 
make allowance for differences of formation, and in that way obtain a general conception 
of the distribution of intensity with reference to the fault and the Rift. 
The intensity was greatest on the line of the fault, but did not diminish rapidly toward 
the east and west within the Rift belt. In a general way the intensity was greater in the 
Rift belt than on either side. On the east it fell off rapidly — almost suddenly — at 
the limit of the Rift. On the west it fell off gradually, being nearly as high at a distance | 
of 0.5 mile or 0.75 mile from the rift as at the edge of the Rift. In a general way the 
intensity west of the Rift was greater than at the east. My conception of the distribu- 
tion on a line normal to the Rift is exprest by the following curve (fig. 50), but this 
should not be subjected to measurement, as 
its elements are not definitely quantitative. 
It is a generalization from data that are Principal_| Rift 
heterogeneous and by no means complete. ape | 3 é 
Ina general way the distribution of high Fic. 50. — Curve illustrating distribution of intensity in 
A Ns = 2 ° relation to fault-trace and Rift. The height of curve 
intensity follows the distribution of bed- above horizontal line represents intensity. 
rock cracks. Inverness, where the injury 
to structures on firm ground reached a maximum, is traversed by important bedrock 
cracks, some of which are to be accounted as branches of the main fault. The high 
ridge west of the main valley, over which the intensity was nearly as great as along the 
Rift, was also characterized by many important bedrock cracks, and by a general 
derangement of the underground circulation of water. The district east of the Rift, 
where the intensity rapidly diminished, was practically exempt from bedrock cracks, 
and its underground circulation was not disturbed. 
Notes by other observers (R. 8. Holway).—A bridge about 0.75 mile southeast of 
Point Reyes (toward San Francisco) went completely down, causing several days’ delay 
to trains. The track had had several horizontal bends of a few inches. 
The “fills”’ across the arms of Tomales Bay generally sank from 2 to 8 feet. The 1,000- 
yard fill about 2 miles north of Point Reyes Station sank from 6 to 8 feet; as did the next 
fill, which is some 500 feet long. In one or two instances the pile-supported bridge in 
the middle of the fill remained at grade. Just-above Hamlet a trestle-work which had 
been filled in settled, leaving the trestle-work some 2 feet above. ‘The bottom of the bay 
in these arms is usually sand. 
At Hamlet quite an extensive landslide has started in the hillside above the track. 
The railroad cut is in old rock, and the arch of the head of the slide is some 70 feet above 
the track. The country wagon road has been carried away by the slide for possibly 100 
ards. 
J Miss Margaret Keating, a teacher at Marshall’s, just at the close of the earthquake saw 
two waves coming from the opposite side across the Bay; that is, the length of the wave 
was parallel to the main Rift. The waves were from 6 to 8 feet high. ‘The waves came 
nearly to the top of the trestle, and also up to certain willows which she indicated, both 
points roughly indicating a wave of the height she mentioned. 
At Marshall’s a hotel and a stable built on the west side of the track and on under- 
pinning, resting in the tidal flat, went easily and gently into the bay. The occupants of 
the hotel did not realize that the hotel had fallen, but at first thought the water had 
risen. At the post-office store goods were thrown from the west wall, but scarcely at all 
from the east. 
sw NE 
