﻿350 DR. J. W. EVANS ON [Aug. !9 IO > 



Such a movement may be explained without difficulty, on the 

 hypothesis that the earth as a whole is subject to powerful tan- 

 gential compression. This view has, it is true, been called in 

 question, on the ground that contraction of the earth's interior due 

 to cooling is insufficient to account for the folding and overthrusts 

 which can be shown to have taken place. It must be remembered, 

 however, that the interior must also diminish in volume as a result 

 of volcanic eruptions and loss of water and other materials from 

 springs of intra- telluric origin. At the same time, the crust is 

 always expanding in consequence of the gradual hydration of its- 

 crystalline rocks, which probably constitute by far the greater 

 portion of its mass, by water both of intra- telluric and of atmo- 

 spheric origin. 1 Not only does this take place in the great tracts 

 of the fundamental gneiss, but every intrusive boss or dyke must 

 exercise expansive action as its alteration proceeds." 



The Pacific coast of North America constitutes a line of weakness 

 connected with the folding that gave rise to the coastal ranges. 

 Jn the extreme north of the Pacific, in the neighbourhood of the 

 Aleutian Islands, this changes from a south-east and north- 

 west to an east-and-west direction. In North America, on the 

 other hand, there is no transverse — that is to say, east-and- 

 west — line of weakness in the north, but to the southward 

 we have one stretching through the Antilles and Mexico. Similar 

 relations prevail on the Asiatic side. Accordingly, as the crust 

 adjusts itself by folding and thrusts where it is weakest, a north- 

 ward movement of the bed of the North Pacific relatively to both 

 North America and Asia may be expected to take place. This is r 

 in fact, what occurs, for a relative movement to the north can be 

 shown to have taken place not only on the west side of the great 

 Californian fault in 1906, but also on the east side of the well- 

 known Neotale fault in Japan at the time of the great earthquake 

 of 1891. 



This gradual northward movement of the North Pacific relatively 

 to the continents on either side must be accompanied by intense 

 folding and thrusting in the neighbourhood of the Aleutian Islands; 

 and immediately to the south of these is one of the most active 

 earth quake- tracts of which we have any knowledge. At the same 

 time, the lines of weakness on the borders of the continents must be 

 affected to some extent in a similar manner. It is true that in the 

 case of the earthquake of 1906 the movement was mainly longi- 

 tudinal parallel to the coast, but there were not wanting indications 

 of a certain amount of overthrusting of the continental over the 

 marine area. 



1 It is probable that pneumatolytic action in general usually causes expan- 

 sion : for instance, in tbe conversion of a granite into a greisen. 



2 I need not say that there are other causes, such as tidal action and changes 

 in rock-temperature, which may in places be responsible for variations 

 of tangential pressure or the local occurrence of conditions of tension, and 

 the latter undoubtedly also appear in the course of the development of great 

 flexures in the earth's crust. 



