144 S. SEKIYA AND Y. KIKUCHI 



or black colour, which some observers have referred to a molten 

 origin. It is more than probable, however, that these rocks are the 

 fragmentary materials which, having been ejected during the pre- 

 vious periods of activity in the history of the volcano, had accumulated 

 in the form of volcanic strata, and taken their share in the building- 

 up of the mountain. The destruction of Kobandai, which was itself a 

 part of the side-wall of the volcano, reduced these strata to a powdery 

 state, and scattered the scoriaceous materials imbedded in them abroad 

 on the mud-field. 



Survey of the Crater and the Volume and Weight of the 

 Mountain destroyed. 



After great volcanic eruptions, surveys have sometimes been 

 made with the view of estimating the dimensions of the parts blown 

 away, or the amount of material ejected during the outbreak. The 

 difficulties attending these attempts are obvious, for it happens very 

 often that the seat of the outburst is inaccessible on account of lava 

 flows or other dangerous obstacles that beset the way. 



In estimating the dimensions of the newly opened explosion- 

 crater which was formed by the destruction of Kobandai, and in 

 deducing therefrom the volume and the weight of the mass that 

 was thus removed, we encountered one serious difficulty, viz., igno- 

 rance of the original topography and former contour of the moun- 

 tain. x\n unexpected advantage was however derived from the fact 

 that, since in the case of Bandai the explosion lasted a very short 

 time and no subsequent outbreaks took place, we could walk into 

 nearly every nook and corner of the newly formed crater, and thus 

 with comparative ease complete a survey which might otherwise have 

 been almost impossible. The inaccessible or dangerous parts were the 



