230 EAKTHQUAKES. 



than real. Thus the north and south ranges of mountains 

 in Japan are mountains almost simultaneously shaken 

 along their eastern flanks, giving the impression that an 

 earthquake had originated simultaneously from a fissure 

 parallel to this line, or else, starting at one end, had run 

 down their lengths. Time observations have, however, 

 shown that such disturbances had their origin at some 

 distance in the ocean, and, travelling inwards, had reached 

 all points on the flanks of these mountains almost simul- 

 taneously. The same explanation will probably hold for the 

 so-called linear disturbances of western South America. 



All earthquake disturbances have probably a tendency 

 to radiate from their source, and are only prevented from 

 doing so by meeting with heavy mountainous districts, 

 which by their mass and structure absorb the energy com- 

 municated to them. Much energy is also lost by emerg- 

 ence on the open flanks of a range of mountains. Kather 

 than say that high mountains often bound the exten- 

 sion of an earthquake, or that earthquakes appear to run 

 along the flanks of such mountains, we might say that 

 earthquakes have boundaries parallel to the strike of the 

 rocks in a given district, that such a direction is the one 

 in which the propagation is the easier. 



Eossi is of opinion that volcanic fractures play an 

 important part in governing the distribution of seismic 

 disturbances. When a volcano is formed, a series of 

 starlike fractures are formed round the central crater. 

 Secondary craters may indicate the line of these fissures. 

 The mountains about Eome are regarded as typical of this 

 radial structure. The more distant the secondary craters 

 are from the centre of the system, the smaller will they 

 he, and also the younger. If two fissures intersect we 

 get a larger crater at the junction. Earthquakes are 

 propagated along the direction of these fissures, whilst the 



