Iviii PEOCEEDIIS^GS OF THE GEOLOGICAL SOCIETY. [vol. IxXVlii, 



the solid outer crust of the Earth, and in this region it is difficult 

 to conceive of any cause, sufficient to originate the elastic wave- 

 motion of the earthquake, other than the sudden fracture of the 

 solid rock, Avhere strain has outgrown the power of resistance. 



Apart from this general reasoning from observation, there are 

 eases on record where considerable displacements of the ground 

 have been measured by the comparison of careful and accurate 

 surveys made before and after the earthquake. In three of these, 

 the Cutch earthquake of 1819, the Sumatran of 1892, and the 

 Californian of 1906, the largest movements took place close to 

 the line of fracture, and in opposite directions on opposite sides of 

 it, the displacements decreasing on either side till a region was 

 reached in which no change from the condition before the earth- 

 quakes could be recognized. As this is precisely what would 

 take place if a solid body, capable of elastic deformation, Avas 

 strained until fracture took place, the conclusion is justifiable that 

 such was in fact the origin of the dislocation and displacements. 



To this general rule there are some recognized exceptions. 

 Quakings of the earth, identical in character with the true earth- 

 quake, may be caused by natural rockfalls, or artificially by 

 explosions; and there is the class of volcanic earthquakes, recognized 

 as due to disturbances directly connected with volcanic activity, 

 though it is probable that the majority of volcanic earthquakes 

 are in reality originated by rock-fracture and, therefore, only 

 indirectly the result of volcanic activity. To these may, perhaps, 

 be added earthquakes which are due to the direct trans- 

 mission of elastic wave-motion from the bathyseism; but, when all 

 these allowances are made, it must be admitted that, speaking 

 generally, the immediate cause of earthquakes is the development 

 of a state of strain in the outer rocky crust of the Earth, of such 

 magnitude as to give rise to fracture, accompanied or not by 

 displacement of the opposite sides. 



So far the conclusions, which may be drawn from observation 

 as they have been briefly outlined, belong rather to the domain of 

 physics than of geology ; but, when Ave go on to consider the cause 

 to which the strain is to be attributed, and more especially the 

 rate of growth, we are brought into contact with problems and 

 deductions which are intimately connected with geology proper, 

 and to which I propose to confine attention in the remainder of 

 this address. As regards cause ; this is usually attributed to what 

 are known as the tectonic processes, a term Avhich has never been 



