386 SEE— THE CAUSE OF EARTHQUAKES. [October 19 



violent earthquakes occur at greater depths, and no one can predict 

 the inequahties of the encountered resistance. It would all depend 

 on the shape and relative situation of the expanding matter in rela- 

 tion to the surrounding rock, which would also be at high tem- 

 perature. 



As the reservoir of steam-saturated lava would rarely be spherical 

 and would usually be a layer, as we have seen in part IV, and several 

 separate and distinct reservoirs might develop near one another, 

 a streaming of the released matter, when the walls yielded and the 

 strain is released and the fluid quickly adjusted itself to the new 

 surroundings, would almost always produce some kind of rotatory 

 motion, and it would always lie in a plane between the horizon and 

 the zenith. If the layers were complex or of irregular figure this 

 movement might be made up of several parts and the adjustment 

 occupy several seconds, and possibly minutes, of time, even when 

 the forces are enormous and the motion correspondingly rapid. 



The successive powerful impulses or blows imparted to the sur- 

 rounding earth might be of unequal intensity and not all in the same 

 plane, and moreover the vibration would continue for a short time 

 after the internal movement had ceased, on account of the elasticity 

 of the rocks of the earth's crust. Lava saturated with superheated 

 steam would behave essentially like steam in an exploding boiler, 

 because it would give body and momentum to the spreading steam 

 and be capable of transmitting shocks of appalling violence. 



This gives us a conception not only of the process involved in an 

 earthquake, but also of how the irregularities noted by seismographs 

 might be accounted for; and when we recall that the subterranean 

 boiler might surpass the largest mountain in size, or be flattened 

 into an immense disc^ of slight thickness, with vent chiefly or 

 wholly at the sides, we can easily understand the terrific forces 

 which shake the whole earth when once the surrounding walls give 

 way or a fault moves, so that an explosion and diffusion of the lava 

 is effected. 



It must be assumed, for reasons already fully developed, that 

 sensible readjustment and motion of large masses of steam-saturated 



^ It is found by investigation that many of the tectonic earthquakes origi- 

 nate in an area of considerable extent. 



