part 1] ANNIYERSAEY ADDRESS OF THE PRESIDENT. llX 



defined and is incapable of precise definition, but may approximately 

 be described as the processes by which the folding and faulting 

 of rocks were produced, and, in accordance with this attribution, 

 the class of earthquakes, with which we are concerned, is referred 

 to as tectonic . The rate]of growth of strain has almost invariably 

 been accepted as very slow, yet, when the subject is looked into, 

 it will be found that there is really no evidence to support the 

 acceptance ; in part, it must be attributed to the general belief 

 that all geological action is necessarily slow, and in part to the 

 conclusion, forced on us in the latter days of the last century, that 

 the Earth is a solid, inert, and highly-heated body, cooling slowly 

 by radiation, with the subsidiar}^ deduction that all deformation 

 of the outer crust must be referred to contraction, consequent on 

 that slow cooling. The latter of these reasons is now abandoned 

 by those who forced it on us, and the foi-mer, though true in 

 general, must not be treated as an unchangeable law, for there are 

 many cases where a process, slow on the average, and as a rule, 

 is occasionally subject to a temporary acceleration of rate. The 

 evidence, too, which has been regarded as confirmatory of the slow 

 growth of strain, may more properly be described as an interpre- 

 tation of observed facts in accordance Avith an hypothesis. 



In the report on the Californian earthquake of 1906, for instance, 

 the displacements caused by that earthquake, and an earlier one 

 in 1868, are discussed, and the conclusion is drawn that they 

 should be explained by a slow growth of strain, extending over a 

 century or so, partly relieved hy fracture in 1868, and again in 

 1906. The argument is conclusive, in so far as it shows that the 

 effects are consistent with the hypothesis ; but it was not noticed 

 that they would be equally consistent with a condition of quiescence 

 throughout the whole period, with the exception of two short 

 intervals immediately preceding the two shocks, respectively. 

 The same may be said of all the supposed evidence in favour of a 

 slow growth of strain ; the after-effects may satisfactoril}^ establish 

 the conclusion that the proximate cause is fracture resulting from 

 excessive strain, but they can give no indication of the time 

 occupied in preparation. The earthquake comes and passes, it 

 leaves certain records behind it ; but these records would be the 

 same, whether the preparatory growth of strain was secular or 

 instantaneous in duration. 



Yet the problem is not insoluble, for there is another line of 

 attack, which has only become practicable within the last few 

 years. If we regard the growth of strain as continuous, there will 



