382 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



subsidence. Yet it is evident that these faults may easily result from 

 the mighty steam pressure which has shaken the earth sufficiently to 

 break the layers of rock, and give relief to the pent-up forces. In 

 so-called tectonic earthquakes the forces usually are greater than 

 those involved in volcanic action, because the whole body of the 

 overlying strata must be shaken to afford the smallest relief, but are 

 seldom so concentrated as would be required for the upheaval of a new 

 volcano through the entire depth of the unbroken crust. If the crust 

 is already badly fractured or breaks more easily than it gives relief 



Fig. 16. 



Z^nh xiaKen Strata . 



when shaken, sometimes new volcanoes actually break forth, especi- 

 ally under the sea, where the explosive forces are greatest. 



When we consider the terrific shock required to break all the 

 horizontal strata in a situation where the imprisoned forces are 

 deep-seated we can readily imagine that yielding will often occur 

 through condensation of soft rocks, through tightening up of crev- 

 ices, joints and faults, and sometimes by uplifts of all the strata, 

 affording room for the injection of a layer of lava of large extent, 

 or its diffusion by spreading into surrounding areas. The move- 

 ment of the fluid is shown by the arrows in the figure. In that case 



