1921] Lawson : The Mobility of the Coast Ranges of California 433 



San Andreas rift. It is probable from direct observations of relative displace- 

 ment along the fault-trace in 1906 that the southwesterly block moved north- 

 west 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 certain earlier displacement of Lonia Prieta. Accepting the southerly 

 character of this earlier movement as certain, there is forced upon us the 

 remarkable fact that the direction 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 movement 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 

 differentially 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, without known rupture, at 

 Monterey Bay and San Francisco Bay, cannot at present be stated. 



In the interval between the issue of volume I of the Report of the 

 State Earthquake Investigation Commission in 1908 and that of 

 volume II, containing Reid'.s discussion, Rothpletz* published a paper 

 entitled "tiber die Ursachen des Kalifornischen Erdbebens von 1906," 

 in which he discussed the data contained in volume I of the Earth- 

 quake report, and advanced a theory explanatory of the cause of the 

 disturbance. 



In this paper Rothpletz considers that the results of the geodetic 

 survey, set forth by Hayford and Baldwin, establish the fact of disten- 

 sion of the region in which San Francisco Bay lies ; and his account of 

 the mechanics of this distension is his theory of the cause of the earth- 

 quake. He discusses first a distension which affected the region prior to 

 1906, presumably in 1868, at the time of the great earthquake of that 

 year, and considers three hypotheses: (1) Tangential pressure, or 

 expansion of a compressed region by reason of the diminution of pres- 

 sure. (2) Expansion due to increase of temperature. (3) Injection of 

 dykes of molten magma. He rejects the first two of these hypotheses 

 and adopts the third. He holds that the apparent distension was due 

 to the injection into the crust of a groiip of dykes transverse to the 

 direction of maximum movement, that the aggregate width of the 

 dykes was 7.2 meters, and that the injection was effected suddenly, 



* Sitzungsberichte, Kon. Bayer. Akad. d. Wissen., Miinehen, Math.-phys. 

 Klasse, 1916, 8; Abhandlungen, 1910. 



