AND VALPARAISO EARTHQUAKES AND THEIR CAUSES. 49 



naturally raises. It leaves such discussion for a more exhaustive 

 report, which can only be prepared after the campaign of data 

 collection is complete, and that may be some months hence. ""^^ 



Previous Earthquakes of California. 



During all the Pleistocene and recent periods of geologic 

 history the area of California, and indeed of all our I^acitic coast, 

 has been frequently shaken by less or more severe earthquakes. 

 Professor Edward S. Holden has catalogued them from 1769 to 

 1896, recording at least ten shocks of as great intensity as the 

 last in the region of San Francisco. 



The most severe shock in all this list was that of Owen's 

 Valley or Inyo, about 275 miles east-south-east of San 

 Francisco, on March 26th, 1872. Its fault line was near the 

 steep east border of the Sierra Nevada, and the surface rocks 

 and soil were broken along a distance of forty miles from north 

 to south, with displacement of the side adjoining the mountain 

 range, as compared with the other side, from 5 or 10 to 25 feet 

 of vertical uplift. 



In the thirty-seven years from 1850 to 1886 inclusive, 

 Holden's catalogue shows 254 noticeable earthquakes in San 

 Francisco, or an average of seven yearly. During the same 

 time no less than 514 other earthquakes, not noticed in San 

 Francisco, were felt in other parts of the State. 



Summing up the characteristics of the California earthquakes, 

 Dutton writes : 



"High intensities are not common. The lighter intensities are 

 felt over considerable areas, which suggest great depth of focus. 

 The seismographic traces show consideral)le length of period and 

 well-marked separation between the short preliminary tremors and 

 longer waves, which is indicative of considerable distance travelled 

 by the vibrations between the centrum and the recording station. 

 The deep foci, the long periods, the absence of small tremors, the 

 considerable areas over which light vibrations are felt, are indicative 

 of tectonic rather than volcanic origin." 



* A very interesting and detailed account of the Californian earth- 

 quake of i906 is given by Professor T. W. E. David, of Sydney 

 University, in the Sydney jDailt/ Telegraph of December, who visited 

 San Francisco shortly after the conflagration. His account closely agrees 

 with that of the author.— Editor. 



