ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 349 
ground in which the shock was of exceptional severity. It was traversed by numerous 
cracks, and there are other manifestations of acute disturbance of the ground, as set 
forth in the more detailed section of the report. In this case it is quite possible and 
even probable that the movement on the main fault in the line of the Rift was distributed 
to some slight extent along the branch fault. It is to be noted, in this connection, that to 
the south of Black Mountain there is a slight curvature in the course of the main fault 
to the eastward. This curvature would present an exceptional obstacle to the move- 
ment of the two crustal blocks, the one on the other, greatly increase friction, and so 
locally intensify the shock. It may thus be that the exceptional intensity in the Black 
Mountain mass, and the consequent bulging of the isoseismals on either side of the fault 
in this vicinity, is referable to this irregularity in the plane of the fault; and that the 
branch fault at Portola may be a means of relief from the excessive pressure locally in- 
duced by the irregularity. On the southwest side of the San Andreas Rift, and on the 
other side of the bulge in the fault-trace, is the Castle Rock fault, the strike of which 
branches from the main fault on the Rift at an angle of about 20°. Altho this fault has 
not been actually traced into the line of the Rift, there can be little doubt that it is a 
branch from that fault-zone and it probably bears the same structural relation to it that 
the Black Mountain fault does, 7.e., it serves as a means of relief for the exceptional 
local pressure due to the nearby irregularity in the main fault. There is, however, no 
observational evidence of any movement having occurred on the Castle Rock fault on 
April 18, altho it lies within the region of bulging isoseismals. 
In the Mount Hamilton Range, between Niles Canyon and Mount Hamilton, there 
are many faults; but none of them, so far as the information available will warrant a 
conclusion, appears to have affected in any way the distribution of intensity. Two of 
these, the Mission Peak fault, which is probably a branch from that on which cracks 
opened in 1868 near Haywards, and Mission Creek fault, pass close to the town of Niles. 
But the apparent intensity at Niles was less than on the flat alluvial tract to the west, 
and not greater than in the valley-land about Pleasanton and Livermore to the east ; and 
this circumstance amounts to a proof that no movement occurred on either of these 
faults. A similar conclusion may be drawn with reference to the Sunol fault, from the 
fact that the apparent intensity at Sunol was somewhat less than at Pleasanton, altho 
the former is nearer the Sunol fault than is the latter. In the country between the 
Haywards fault and the Sunol fault there are several minor faults, but there is no indica- 
tion in the distribution of intensity of movement having occurred on any of them. 
Similar statements are true of the fault zone extending from the vicinity of Benicia 
northward on the west side of the Sacramento Valley. 
In the canyon of Pajaro River, below Chittenden, there is an east-west fault whereby the 
Tertiary rocks on the north side have been dropt against the granitic rocks of the Gavilan 
Range on the south. This fault crosses the San Andreas Rift, and its known extent on 
either side of the Rift is within the zone of high intensity referable to the movement 
of April 18. There are here no especial features in the distribution of apparent inten- 
sity which suggest any movement on this fault. It is possible, however, that a slight 
movement took place on this fault, since the steel bridge over the Pajaro River, which 
is about on the intersection of the two faults, was distended 3.5 feet between its end piers, 
as shown in plate 65n, ina way that can not be altogether satisfactorily explained by the 
movement on the fault along the Rift. The direction of the chief displacement of the 
piers was about midway between the strikes of the two faults. 
In the Santa Lucia Range to the southwest of the Salinas Valley, there are several 
faults. The principal one runs along the northeast flank of the range on the edge of 
the Salinas Valley. The reasons for ascribing the high apparent intensity on the floor 
of the Salinas Valley to the character of the underlying formations, rather than to any 
