Ixiv PEOCEEDIXGS OP IKE GEOLOGICAL SOCLETY. ~Vol. Ixxviii, 



that, in order to reach this point in one rear from starting, the 

 strain would have to increase at the rate of about 1400 grammes per 

 centimeti-e square in each quarter of a day. Accoixlins: to the late 

 Su* Greorge Darwin the stress-differences set up by the moon in the 

 latitude of Italy would amount to about 20 grammes per centimeti-e 

 square in an incompressible Earth, and in a compressible Earth ^vith 

 an incompressible crust — a condition much more akin to what we 

 have reason to suppose is the reality — the stress-differences would 

 be many times this figm-e ^ : but even the lower amount is nearly 

 1| per cent, of the gi-owth requii-ed to reach breaking point in one 

 year, it would be close on 15 per cent, if the period is increased 

 to ten years, and, with anything approaching this proportion, a 

 periodicity would result, which could not have escaped detection 

 before now. 



The figures, therefore, give us a lower limit of the rate of 

 growth of strain, it must have been something faster than that 

 needed to reach the breaking point in one year fi-om starting, if the 

 differences on which the argument is based are real. But are they 

 real? The actual amount of difference, barely 1 per cent, of 

 the mean, is so small that it may well be fortuitous, and the true 

 interpretation may l^e that the gravitational stresses, and the 

 stress-differences produced by them, have no effect whatever in 

 determining the time of occurrence of an earthquake. If this be 

 so, then the rate of growth of strain becomes infinite, and each 

 earthquake becomes the result of a rapid development of strain, 

 akin to an explosion in its suddenness. 



The truth may lie anywhere and must lie somewhere between 

 these extremes, and so we reach the conclusion that there is no 

 support for the commonly-accepted notion of a continuous, slow 

 gi'owth of strain, extending over years, decades, or even centmies, 

 before the breaking point is reached. On the contrary, it appears 

 that the cause of earthc[uakes is a rapid growth of strain. This 

 strain cannot be developed Avithout some deformation, but the 

 magnitude of this has no relation to the frequency or magnitude of 

 the earthquake ; if change of form is slow and prolonged, relief 

 mav be provided by gradual yielding, if rapid, a very small amount 

 of distortion may lead to fi-acture, and on the extent, form, and 

 position of this fracture will depend the character of the resulting 

 earthquake. 



This study of the growth of strain leads on to the Cjuestion, 



1 Sir G. H. Darwin. Scientific Papers, vol. ii (190S) p. 502; and Plul. Trans. 

 Eoy. Soc. vol. clxxiii (^1SS2) pp. 219 et seqq. 



