288 Y. OINOUYE 



southeast and elliptical in plan, took place in the midst of the 

 activity. Of course, we must not forget that the elevation is not 

 entirely due to the intrusion of the plug, but that there was co- 

 operation of the faulting such as is so remarkably shown on the 

 top of Kompirayama. The elevation of the lake shore on the 

 south side of Lake Toya may be accounted for by the tilting of the 

 crust owing to the intrusion of the plug rather than, as previously 

 supposed, to the formation of a laccolith (Fig. 27). From dynamic 

 considerations it is evident that if the force of the plug intrusion be 

 applied to one end of a resistant section of the earth's crust the 

 whole block will be lifted and tilted. 



How could such a very steep slope (40°-7o°) on the south side 

 and 22°-30° on the north side be made on the surface by the intru- 

 sion of a laccolith ? If we suppose that there is a very sharp, steep 

 dome in the great depth overlaid by heavy layers above, its inclina- 

 tion becomes gradually gentle toward the surface, unless the crust 

 be in the liquid or semiliquid condition. 



4. Undulation of ground near Usu.- — From the damage done, and 

 from faults, fissures, and mud cones which were found exclusively 

 on the same side, we may prove that the structure of the western 

 region was originally weak, so that the shaking was intense, while 

 in other parts the effects were comparatively small. Frequent 

 explosion also weakened the already feeble lines. 



We may then conclude, from evidence gathered in the field, that 

 there must have been intruded irregular bodies of lava that pro- 

 duced the undulation of the ground near Usu recently observed in 

 the eruption. 



