AND VALPAKAISO EAKTHQUAKES AND THEIK CAUSES. 55 



the former river gorge into the present magnificent " Golden Gate." 

 So clearly had the geotectonic structure of the region been worked 

 out by Lawson, that the speaker had ventured in a public address 

 (within a fortnight of the occurrence of the earthquake) to offer with 

 some confidence what seemed to him — as a deduction from Dr. 

 Lawson's paper — to be the true explanation of the earthquake, in 

 the sudden relief of strain in the bed-rocks of the region, along a 

 jjlane or planes of slip-faulting, which are generally found in the 

 synclinal portions of all great flexures of the earth's crust. 



In the interim " Report " of the Commission, which reached him 

 three or four weeks later from Dr. Lawson, he had the satisfaction 

 to find that the explanation he had suggested agreed with that put 

 forward by the Commission as given in Dr. Upham's paper. 



The speaker regretted that Dr. Upham had not been able to give 

 more detailed information, at present, as to the nature of the crustal 

 movements, which had operated with such disastrous results in 

 the Valparaiso region ; but he strongly suspected that further 

 investigations might result in interpreting those upward movements 

 mentioned b}^ the author, as indicating " over-thrust " fault-planes 

 due to the fact that the region in question is situated on the ridge 

 of an anti-clinal fold. If that should turn out to be the case, the 

 fact would probably account for the wider extension of the 

 disastrous results there than in the San Francisco region. 



Dr. Irving ventured to put before the Meeting an interesting- 

 problem, which had presented itself to his mind, as to a possible 

 connection in time between the occurrence of the San Francisco 

 earthquake and the great eruption of Vesuvius about a week before, 

 with the abnormally extensive extrusion of lava from the depths at 

 which molten or potentially-liquid rock-material exists.* The 

 latitude of Vesuvius and of San Francisco being nearly the same, 

 and approximately 40° N., a simple calculation gives us a rapidity 

 of rotational movement from west to east along that zone of 

 latitude of something like 700 miles per hour (about ten times 

 greater than the velocity of the fastest express train) to represent 

 the 1,000 miles per hour velocity of rotation of the outer rind of 

 the earth in equatorial regions. The extrusion of lava at Vesuvius 



* On this point reference may be made to the speaker's letters to 

 Nature in May, 1905, vol. Ixxii, pp. 8 and 79. 



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