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



Another way to reach this result is to recall that the investiga- 

 tion of earthquakes in Hungary, Croatia, Bohemia and other coun- 

 tries shows that most frequently the impulse proceeds from an 

 underlying area of considerable extent, and not from a point nor 

 line, though the surface movement may be chiefly along the nearest 

 fault. If the underlying area from which the shock proceeds is 

 elliptical, as usually happens, we may be quite sure that this diffusion 

 of the impulse over an area indicates an underlying outspread sheet 

 of lava saturated with steam, which finally acquires such tension that 

 it is enabled to shake the overlying crust and cause a movement 

 along the fault lines where the crust is broken and movement is 

 easiest. The lava sheet seeks readjustment, and in the process of 

 equalizing the strain, movement of the molten rock takes place, and 

 the faults not only move vertically, but often also horizontally. 

 This is a natural and simple explanation of fault movement, and it 

 accounts for the rotatory motion so frequently noticed in earth- 

 quakes. The great earthquakes of Lisbon, Arica and Iquique have 

 usually been classed as tectonic, but in view of the sinking of the 

 bed of the sea shown by the accompanying seismic sea waves, it is 

 clear that all these terrible disturbances were due to the expulsion 

 of lava from under the ocean. 



§ 55. The geological significance of earthquakes. 



In their new work on " Geology " (Vol. I, p. 534) Chamberlin 

 and Salisbury remark that '' Earthquakes are of much less im- 

 portance, geologically, than many gentler movements and activities. 

 Disastrous as they sometimes are to human affairs, they leave few 

 distinct and readily identifiable marks which are more than tem- 

 porary." Mention is made of the effects of earthquakes in fractur- 

 ing the rocks of the earth's crust, but the fractures, it is pointed out, 

 do not show at the surface when the soil is deep. These authors 

 also remark (p. 527) that " the most prevalent (source of earth- 

 quakes) is probably the fracture of rocks and the slipping of strata 

 on each other in the process of faulting." 



Let us now examine these remarks a little more carefully. If 

 earthquakes are due to fracture of rocks and the slipping of strata, 

 it follows that the forces involved here have played no part in the 

 original formation of the globe, but are the effects of collapse after 



