520 ROBERT ANDERSON 



ing the whole upper part of a mountain of such size and leaving 

 such regular remains, whereas they might well be the outcome of a 

 comparatively gentle process of sinking taking place simultaneously 

 at all points around a common center. In the second place, the 

 great lava flows radiating from the volcano are the quantitative equiva- 

 lent of the mass that has disappeared from the old cone of Mount 

 Aso, and may be reasonably considered as derived from the space in 

 which the mountain became engulfed, having vacated this space 

 through channels that opened at low points on the flanks of the cone. 

 In the third place, the lava, although it is not of the most fusible 

 variety, is of such a character that it would be capable of forming 

 large, easily flowing and slowly cooling streams; and would not 

 be of such viscosity as to offer the utmost resistance to explosive 

 forces and to cause explosions of the greatest magnitude. 



As regards the possibility of an explosive origin for the crater, it is 

 to be expected that a prominent rim of debris would have accumulated 

 around the brink of the cavity had such an immense block of the 

 earth's surface been removed in this manner. Although it is true that 

 a vast amount of fragmental material is present in the wide region 

 surrounding Aso-san, no such rim is to be found. The balance of 

 probability favors the conclusion that upward discharges of material 

 did not play a chief part in excavating the bowl, but that the masses 

 of ejectmenta covering, much of the region around the volcano were 

 thrown out partly before the destruction of the ancient mountain and 

 partly during the period in which the process of engulfment was 

 progressing, as a secondary phenomenon attending that catastrophe. 

 The strata in the wall tell of explosive eruptions that took place 

 contemporaneously with the emission of lava streams in the early 

 history of the volcano, but it appears that the erupted material was 

 chiefly in the form of lava flows. On the other hand, the decadent 

 stages of the volcano have been marked chiefly by explosive eruptions 

 as witnessed by the new cone and the deposits in its vicinity, and by 

 the historical accounts. 



As regards the other hypothetical mode of origin, the low cone of 

 Aso may be thought of as having been formed by a process of cone 

 building, through the overflowing of lava from a very large vent 

 and the accumulation round about of material explosively removed. 



