82 EARTH FEATURES AND THEIR MEANING 



It is thus possible upon this assumption crudely to demonstrate 

 the adjustment of earth blocks by the simple device represented in 

 plate 4 A. The construction of this experimental tank is so simple 

 that little explanation is necessary. Wooden blocks of different 

 heights are supported in water within a tank having a glass front, 

 and are kept in a strained condition at other than their natural 

 positions of flotation by the compression of a simple vice at the 

 top. Held firmly in this position, they may thus represent the 

 neighboring blocks within the earth's outer shell which are sup- 

 ported upon relatively yielding materials beneath, and prevented 

 from at once adjusting themselves to their natural positions through 

 the compression to which they are subjected. Held as they now 

 are, the water near the ends of the tank is forced up beneath the 

 blocks to higher than its natural level, and thus tends to flow from 

 both ends toward the center. Such a movement would permit 

 the end blocks to drop and force the middle ones to rise. The end 

 blocks are, let us say, the sections of Alaskan coast line which sunk 

 during the earthquake, as the center blocks are the sections which 

 rose the full measure of 47 feet. Upon a larger scale the end blocks 

 may equally well be considered as the floor of the great deeps off 

 the Alaskan coast, whose sinking at the time of the earthquake 

 was the cause of the great sea wave. Upon this assumption the 

 center blocks would represent the Alaskan coast regarded as a 

 whole, which underwent a general uplift. 



Though we may not, in our experiment, vary the tendency to 

 adjustment by any contractional changes in either the water or 

 the blocks, we may reduce the compression of the vice, which leads 

 to the same general result. As the compression of the vice is 

 slowly relaxed, a point is at last reached at which friction upon 

 the block surfaces is no longer sufficient to prevent an adjustment 

 taking place, and this now suddenly occurs with the result shown in 

 plate 4 B. In the case of the earth blocks, this sudden adjustment 

 is accompanied by mass movements of the ground separated by 

 faults, and these movements produce successional vibrations that 

 are particularly large near the block margins, and other frictional 

 vibrations of such small measure as to be generally appreciated by 

 sounds only. The jolt of the adjustments has thrown some blocks 

 beyond their natural position of rest, and these sink and rise-sub- 

 sequently in order to readjust themselves with lighter vibrations, 



