ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 041 
tral part of the city and county of San Francisco, 5 to 6 miles from the fault, it was 
about VII. On the alluvium of Mission Valley, at distances of from 6 to 9 miles from the 
fault, the intensity varied from less than VII to between VIII and IX. 
Under similar conditions of ground, the shock was greater nearer the fault; but there 
was much greater contrast between the damage produced by the shock on the summit of 
Telegraph Hill and that in the vicinity of the Ferry Building, at like distances from the 
fault, than there was between the damage near the ferries and that in the immediate 








Fic. 64. — Distribution of intensity in Petaluma. Vertical lines represent area of low 
alluvial land, on which nearly all chimneys were damaged. Horizontal lines rep- 
resent Slopes underlain by rock, on which about half the chimneys were damaged. 
The solid black areas and dots represent exceptionally severe damage. The blank 
area inclosed by dotted lines represents a belt of practically no damage. By R. 58. 
Holway. 
vicinity of the fault. Thus, notwithstanding the general tendency of the intensity 
to diminish with increasing distance from the fault, it seems to be unquestionable that 
the degree of intensity which prevailed at any locality in the city depended chiefly on 
whether the underlying formations are firm rock or incoherent material more or less 
saturated with water. It would even seem possible to discriminate slight differences 
of apparent intensity on different kinds of firm rock for the same distance from the fault. 
Thus the chert hills appear to have suffered less disturbance that those where serpentine 
