5o WARREN UPHAM, M.A.^ D.SC.^ F.G.S.A., ON THE SAN FRANCISCO 



and the escape of vast quantities of super-heated water would tend 

 to create a vacuum below the mountain ; but that result would be 

 prevented by the inflow of the surrounding molten or potentially- 

 liquid material. Taking into account the vast eastward momentum 

 (here pointed out) of the whole mass, dynamical considerations led 

 us to believe that the inflow would be chiefly from the west of the 

 Neapolitan area ; and therefore the power of buoying up the crust 

 would be diminished along a zone of latitude extending a good way 

 in that direction round the globe. Such a disturbance of equilibrium 

 would make itself especially manifest where the conditions of the 

 crust caused local weakness and a tendency to subsidence, such as 

 Dr. Lawson had shown to exist in the San Francisco region. He 

 put this forward as a thesis for discussion, and would be glad if any 

 better mathematician than himself could find a flaw in the 

 argument. 



[It might be noted that the great San Andreas line of fault 

 shown on the map accompanying the paper, running nearly parallel 

 to the Coast Kange, was also shown on a very valuable map, which 

 had been constructed by Professor Branner, of Sandford University, 

 and was published in the Supplement to The Times on December 

 17th, 1906, showing how seismic intensity was centered in and 

 around the San Francisco region as the result of the local 

 instability of the crust.] 



Professor H. Langhorne Orchard, M.A., B.Sc. — Although 

 tectonic earthquakes may be said to be independent of volcanic 

 action, yet there is a connection between earthquakes and volcanoes. 

 Volcanoes form outlets for the accumulated and pent-up energy, 

 thus moderating the violence of the outbreak. The volcano is like 

 the safety-valve of a steam-engine. This explains the fact (referred 

 to by the author) that earthquakes accompanied by volcanic action 

 are less destructive than others. 



With regard to the two great seismic belts (traced out in the paper), 

 it is a relief to note that England is not in either of these belts, 

 though apparently perilously near to the second. Probably we owe 

 more than is generally supposed to the friendl}' vicinity of Iceland. 



The great depth of rupture, in the case of the San Francisco 

 earthquake, is remarkable, and the Investigation Commission are to 

 be thanked for directing attention to the importance, in relation to 

 geophysical science, of studying the question of depth. 



