﻿1060 Geological Society. 



in the form inserted in the Journal. It is hoped that all who 

 intend to subscribe will inform the Institute without delay so that 

 an estimate may be formed of the support which may be relied 

 upon. 



• XCVIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 288.] 



February 17th, 1922.— Mr. R. D. Oldham, F.R.S., President, 

 in the Chair. 



r PHE President delivered his Anniversary Address, the subject 

 *~ of w hie 1 1 was the Cause and Character of Earthquakes, 

 using the word in its original sense of the disturbance which 

 can be felt and, when severe, canses damage, as apart from 

 that which gives rise to distant records, only obtainable by 

 special instruments. The character is sufficiently established as 

 a form of elastic wave-motion, of extreme complexity; this 

 is present in all cases, and may be distinguished as the orchesis 

 of the earthquake. In addition there is, in some cases, a 

 molar, permanent, displacement of the solid rock, which forms 

 the mochleusis. Further it has been shown, definitely in one 

 case and inferentially in others, that, where mochleusis is present, 

 the disturbance of the surface-rocks, to which the earthquake 

 proper can be referred, is only the secondary result of a more 

 deep-seated disturbance, which has been distinguished as the 

 bathyseism. The origin of the elastic wave-motion must be 

 a sudden disturbance of some sort; the depth of origin can, in 

 many cases, be shown to be very moderate, not more than about 

 10 miles, and in this outer portion of the Earth's crust the only 

 sudden disturbance conceivable is fracture, due to strain in excess 

 of the power of resistance. In certain cases such fracture, 

 accompanied or not by displacement, has been recognized at 

 the surface ; and measurements of the displacements show that 

 a state of strain must have existed before actual rupture took 

 place, but give no indication of the rate of growth of the strain. 

 The commonly-accepted notion that the growth must be very slow 

 appears to depend on the assumption that the strain is due to the 

 same causes as those that have produced the folding and faulting 

 of the surface-rocks, and also on the assumption that tectonic, as 

 other geological processes, must necessarily be slow. The problem 

 can only be attacked through the variation in the frequency of 

 earthquakes ; the precautions needed in applying this method 

 are indicated, and, when applied, leave only one existing record 

 available, the Italian one. A discussion of this shows that the 

 rate of growth of strain is, at slowest, such that the breaking- 





