Vol. 6] Reid: The Elastic-Rebound Theory of Earthquakes. 427 



earth's crust was subject to slow continuous movements. The 

 existence of great folds in the rocks undoubtedly contributed to 

 this belief; although the force of this argument does not seem 

 to have been clearly stated. The idea was carried too far, and 

 it was even supposed that the great displacements, which geol- 

 ogists have shown to exist at innumerable faults, were attained 

 by a slow, steady, and long continued movement. 



I am persuaded that besides the sudden fling when there is 

 a slip on a fault, there exist in the crust of the earth slow and 

 continuous movements not uniform, and not even always in the 

 same direction, but still movements which are truly continuous, 

 and have no trace whatever of sudden starts. 



The weakest argument is based on the fact that many known 

 movements of the land are extremely slow and are spread over 

 long intervals of time, and that there are no indications at all 

 that they are simply the sum of a number of small sudden dis- 

 placements. Professor John Milne, in describing the reports 

 giving information regarding the changes of level on the Jap- 

 anese coast, states that some of his informants consider that these 

 changes were due to a great earthquake, "although in no case 

 has it been stated that the changes accompanied such disturb- 

 ances." One might suppose that the continuous nature of the 

 movement could be determined by a series of tidal observations. 

 But many years must elapse before the movement can be de- 

 tected at all, and therefore it is quite impossible to prove by this 

 means that it has been actually continuous. 



The strongest argument is based on the plastic deformation 

 of the rock. The great overthrust faults, the folded and con- 

 torted strata, the existence of slaty cleavage, the thinning of 

 the strata and the flattening of fossils, prove beyond question 

 that the rock has been subjected to enormous pressures. What- 

 ever may be the ultimate cause of this pressure, it is certain that 

 its transmission from one part of the rock-mass to another is by 

 means of elastic forces in the rock; there is always a certain 

 amount of elastic compression or distortion under external force, 

 which indeed determines the amount of force transmitted and 

 is itself determined by the amount of the external force. If 

 a book is placed on the table, there is a slight elastic compression 



