part 1] ANNIYERSART ADDRESS OF THE PRESIDENT, Ivii 



substance of the Earth, not, as was once supposed, merely waves of 

 elastic compression, but of most complicated character, and in all 

 but a small minority of cases nothing but this vibratory move- 

 ment, the orchesis, can be recognized. Occasionally, however, 

 and only in the case of some earthquakes of destructive violence, 

 there is also a bodily and permanent displacement of the solid 

 ground, and this mass, or molar, movement has been distinguished 

 as the mochleusis of the earthquake, as distinct from the elastic 

 displacement, accompanied by return to the original position, 

 which constitutes the orchesis. Noav, the elastic waves can only 

 be initiated by some sudden impulse or disturbance, such as might 

 be produced by the fracture of rock ; and as, in those earthquakes 

 where mochleusis can be recognized, there is usually evidence of 

 sudden movement along some pre-existent fault-plane, or of rending 

 and fissuring of the solid rock, faulting or fracturing has come to 

 be regarded as the cause from which the vibratory disturbance 

 originates. 



This conclusion is supported by the fact that the proximate 

 origin of the shock can almost always be placed at a moderate 

 depth from the surface. It is, unfortunately, impossible to give 

 any precise figures, for none of the methods which have been 

 suggested for determining the depth of the origin can be trusted, 

 some because they depend on assumptions which the progress of 

 knowledge has shown to be erroneous, others because they demand 

 data which cannot be supplied with the requisite precision, if at 

 all ; but there is another way in which some idea of the depth of 

 origin may be reached, based on the fact that there is almost 

 invariably a well-defined area of maximum intensity of shock, 

 surrounded by regions of diminishing intensit}^, as the distance 

 from the central area increases. Since the violence of the dis- 

 turbance will decrease with the increase of distance from the 

 origin, it follows that the nearer the origin lies to the surface, 

 the more closely does the variation of surface-distance from the 

 epicentre approximate to the variation in actual distance from the 

 origin ; hence it is evident that the rate of variation of intensity 

 of the disturbance will give some notion of the depth of the origin. 

 In this way, quite apart from any numerical estimates which have 

 been made, it becomes clear that, excluding a small minoritj'- of 

 earthquakes which will be referred to later, the origin lies at a 

 very moderate depth below the surface, probably seldom over 10 

 miles, and usually less. This places the origin within the limits of 



