THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 151 
1. There was no change in the azimuth of the Diablo-Mocho line. 
2. There was no change in the length of that line. 
3. There was no eaatle change in the relations of these two stations to certain 
others nearer the fault. 
4. The latitude of Ukiah remains the same as before the earthquake. 
The first three of these conditions would be fulfilled if the region including all the sta- 
tions occupied had moved in unison southeasterly with but little or no rotation, a possi- 
bility which it is difficult to deny. The fourth consideration does not preclude this possi- 
bility since the amount of movement involved is probably less than the errors of the 
method used for the determination of the latitude of Ukiah. 
In the region about Monterey Bay the most interesting fact brought out by the geodetic 
resurvey is that the combined effect of the earlier movement and that of 1906 is a southerly 
migration of the earth’s crust on both sides of the San Andreas rift. It is probable from 
direct observations of relative displacement along the fault-trace in 1906 that the south- 
westerly block moved northwest as far as the rupture extended. If this be accepted, then 
the southerly net movement on the west side of the south end of the fault is due to the 
predominance of an earlier southerly movement. This agrees with the positive and cer- 
tain earlier displacement of Loma Prieta. Accepting the southerly character of this 
earlier movement as certain, there is forced upon us the remarkable fact that the direc- 
tion of displacement in the region about Monterey Bay is the reverse of that of the earlier 
movement for the region north of San Francisco Bay. This means that the earlier move- 
ment was distensive in character, displacing the territory to the north of San Francisco 
Bay northerly, and that to the south southerly while the vicinity of the Bay itself was 
relatively neutral. It appears, moreover, that the southerly displacement was differen- 
tially diffused, since the amount of displacement of the south side of Monterey Bay was 
notably greater than that of the north side, resulting in a widening of the Bay by about 
10 feet. 
Similarly the distance between Tamalpais and Black Mountain, both on the same side 
of the San Andreas rift, has been increased by a like amount. The significance of this 
general distension involved in the reversal of the direction of displacement to the north 
and south of San Francisco Bay, and of the differential character of this distension, with- 
out known rupture, at Monterey Bay and San Francisco Bay, can not at present be stated. 
The problem requires prolonged study and repeated measurements to secure the necessary 
data for a proper discussion. It is evident, however, that we are here confronted with 
some of the most interesting phenomena in the mechanics of the earth’s crust, phenomena 
which call for deliberate investigation extending through years and decades anid conducted 
on a wisely planned program. 
