1909.] OUTLOOK OF SEISMIC GEOLOGY. 273 



The common characteristic of all phases of the modern tectonic 

 theory of earthquakes, the evolution of which we have now largely 

 traced, is that the adjustments in position or attitude of sections 

 of the earth's crust are regarded as the proximate cause and not 

 the effect of the shocks themselves. So far as molar movements 

 have been recognized by the advocates of the centrum theory, they 

 have been regarded as the direct consequence of volcanic or explo- 

 sive shocks emanating from a deeper-seated origin. Two recent 

 papers of a somewhat speculative nature, prepared by an astronomer, 

 have sought the cause of earthquakes in a leakage from the bottoms 

 of the oceans.^'' 



The Relation of Earthquakes to Volcanoes. — As already pointed 

 out, the earliest of the generally accepted theories of earthquakes 

 connected them directly with volcanic action, and this idea has sur- 

 vived in the centrum theory. The tendency of later study has been 

 to indicate that while both betray a certain relationship to each 

 other, this is not often of such a nature as to call for a quick response 

 of the one phenomenon to the other. Regions of volcanoes are sub- 

 ject to earthquakes, yet some of the heaviest earthquakes have 

 affected a region distant from any volcanic vents. Again, most 

 great volcanic outbursts are inaugurated by light earthquakes, but 

 great earthquakes produce as a rule no perceptible immediate effect 

 upon the activity of neighboring volcanoes. Thus, for example, 

 during the late Messina earthquake, which was so heavy about the 

 slopes of Etna, that volcano showed no sympathetic response. 

 Catalogues setting forth the seismic and volcanic activity within 

 any province betray, however, certain periods of years during which 

 both seismic and volcanic activity are at either a maximum or a 

 minimum ; though within these periods no close time relation of the 

 one phenomenon to the other is apparent. In short, it would appear 



"T. J. J. See, A.M., Lt.M., Sc.M. (Missou.), A.M., Ph.D. (Berol.), "The 

 Cause of Earthquakes, Mountain Formation and Kindred Phenomena Con- 

 nected with the Physics of the Earth," Proc. Am. Phil. Soc, Vol. 45, 1907, 

 pp. 274-414. " Further Researches on the Physics of the Earth, and espe- 

 cially on the Folding of Mountain Ranges and the Uplift of Plateaus and 

 Continents Produced by Movements of Lava Beneath the Crust Arising 

 from the Secular Leakage of the Ocean Bottoms," ibid., Vol. 47, 1908, pp. 

 157-275. 



PROC. AMER. PHIL. SOC. XLVHL 1 92 S, PRINTED SEPTEMBER "J, I909. 



