288 Transactions. — Geology. 



plosions were caused by "the gradual upheaval of a molten 

 dyke through the upper strata of rocks " — an hypothesis 

 which rests on no evidence whatever. 



It appears, therefore, that there have been in the Hannier 

 Plains two kinds of earthquakes, due to different causes and 

 originating in different places. 



Luminous Appearances in the Sky. 



At Eeefton, in the early morning and in the evening of the 

 1st September, a "luminous appearance" is reported to have 

 been seen in the eastern sky in the direction of Christchurch, 

 and it was again highly visible on the evening of the 8th Sep- 

 tember. _ In Dunedin, on the evening of the 1st September, an 

 extraordinary glow was observed in the western sky, notice- 

 able until after midnight, and it travelled southwards. I 

 mention these things, but I do not think that they were in 

 any way connected with the earthquake. 



Unequal Effects of the Earthquake. 



It is well known that the effects produced by an earth- 

 quake are often apparently capricious. Sir C, Lyell says 

 that in the Calabrian earthquake of 1783 "in some streets of 

 Monteleone every house was thrown down but one, in others 

 all but two ; and the buildings which were spared were often 

 scarcely in the least injured." And many other examples 

 could be given. ]\[uch of this may be due to the different 

 materials of which houses are built, to their different plans of 

 construction, or to their different foundations ; still, when 

 due allowance has been made for all these things, a balance 

 often remains over which can only be explained on the suppo- 

 sition that the shock was actually more severe in some places 

 than in others, irrespective of their distance from the place of 

 origin. More than forty years ago Mr. Eobert Mallet pro- 

 posed a theory to account for these apparent eccentricities. 

 He said, " Where a wave of elastic compression, such as 

 our earth-wave, passes through a body varying in specific 

 elasticity in several parts of its course, or passes from one 

 body to another of different elasticity, at each such change of 

 medium the waye changes its velocity and in part changes its 

 course, a portion being reflected and a portion refracted, 

 analogous to a wave of light in passing through media of 

 variable density or of different refractive indices. "=•' This ex- 

 planation has been universally received as correct ; but it can 

 only be applied to particular cases when the local details of 

 geological structure are well known; and before attempt- 



*" Dynamics of Earthquakes," Pro. Royal Irish Academy, 1846, 

 p. 26. 



