Vol. 6] Eeid: The Elastic-Rebound Theory of Earthquakes. 437 



accumulating the elastic energy, but merely give the modus 

 operandi of the accumulation and liberation of Ibis energy. 



They are opposed to Lieut. Colonel Harboe's idea of focal 

 lines, which assumes that the fracture extends practically as far 

 as the earthquake is felt ; and to the block movements of several 

 writers, who suppose that the earth's crust breaks up into indi- 

 vidual blocks, each of which moves as a whole to a new position of 

 equilibrium. 



It must not be supposed that earthquakes are caused only by 

 horizontal movements on a vertical fracture, as in the case of the 

 California earthquake. Any kind of a fracture is sufficient, and 

 the movements may be horizontal, vertical or oblique. When 

 rocks have been folded in the earth's crust it is not uncommon 

 to find scratches on the limbs of the folds resulting from the 

 slipping of the strata upon each other. Professor Smoluehowski 

 has suggested that this slipping might be a cause of earthquakes. 

 It seems quite certain that, as the rocks were being folded In- 

 horizontal pressure, the friction would at first prevent any such 

 slipping of the strata; but as the elastic forces become stronger, 

 slipping would occur suddenly with an elastic rebound of the 

 adjacent strata, which would constitute an earthquake. Tt is 

 probable that the elastic strains set up in this way and the 

 consequent rebounds would never be very great and, therefore, 

 that severe earthquakes are not originated in this way. 



Let us glance for a moment at the accounts of some other 

 great earthquakes and see if the movements of the ground which 

 accompanied them were similar to those found at the time of the 

 California earthquake, and if the ideas of elastic rebound which 

 we have developed can be applied to these. 



A very severe earthquake occurred in the Province of Cutch, 

 near the mouth of the river Indus, in 1819. An extensive, fiat 

 plain, known as the Rann of Cutch, only a few feet above the 

 level of the sea, and which was indeed formerly a sea bottom, 

 occupies a large area in this region. It is traversed by a small 

 distributary of the Indus, called the Pooraun or Koree, but for 

 some years before 1819 no water had flowed through this channel 

 on account of dams built across it further up. The great shock 



