,9o6.] SEE— THE CAUSE OF EARTHQUAKES. 403 



figure. It will be evident from this consideration and others that 

 the thickness of the crust is by no means uniform throughout the 

 globe. Determinations of the depths of large earthquake disturbances 

 are probably the best means of approximating the thickness of the 

 earth's crust, since data of this kind depend wholly upon observation 

 and are indepenedent of any hypothesis. 



XII. Conclusions. 



§ 62. Summary of results. 



1. We have seen that deposits of sediment on the continental 

 shelves could not possibly produce anything but the most gradual 

 increase of weight on these portions of the earth's crust; and since 

 such rocks as marble are proved to be fluids of great viscosity and 

 therefore capable of slow secular bending without rupture, we may 

 feel sure that any stresses thus arising in the earth's crust would 

 be relieved by gradual yielding, and that no violent earthquake shock 

 could ever arise from such a cause. 



2. The theory that earthquakes are due to fracture and slipping 

 of rocks is disproved by the great depth (ten to twenty miles) 

 at which .world-shaking earthquakes are found to originate, and by 

 virtue of the fact that they come not from a point nor from a line, 

 but from an area; and moreover earthquakes follow the seashore, 

 seldom occurring far inland, and never in desert countries, though 

 abundant in the bed of the ocean. 



3. It therefore follows that earthquakes must depend upon ex- 

 plosive forces within or just under the earth's crust, and frequentlv 

 spread over a considerable area, and the preponderance of disturb- 

 ances in the sea and along the shores of continents shows that the 

 forces depend in some way upon the sea water. These explosive 

 forces can be best studied in connection with the eruption of vol- 

 canoes, since volcanic outbreaks are also accompanied by earth- 

 quakes often felt over large areas. 



4. Not all earthquakes lead to eruptions, but if the shocks in a 

 given region cease on the eruption of a neighboring volcano, we may 

 feel sure that the forces producing the eruption also produced the 

 antecedent earthquake shocks felt by the surrounding country. 



5. That steam is the cause of volcanic eruptions is proved by the 



