THE VOLCANO ASO AND ITS LARGE C ALDER A 521 



But it is scarcely conceivable that a process alone of building up 

 could have produced such forms as those of Aso. 



John Milne discusses the origin of the Aso crater in the account 

 of his visit to it before mentioned,^ He considers unfavorably the 

 theory of an explosive origin and comes to the following conclusion : 



I should be inclined to look upon it as being now, as it ever was, the upper 

 crater of an old volcano, inside which in more recent times a cone has grov/n. 

 Although at the commencement of the mountain the action may have been 

 cataclysmic in its nature, subsequently, hov/ever, I should think that it grew up 

 higher, partly by the accumulation of ashes, but nov/ perhaps by the boiling over 

 of a highly liquid trachytic lava. That this latter action has taken place seems 

 to be testified by the roughly stratified appearances which are exhibited in the 

 ring v/alls; the grov/th has in fact been probably something like the growth of 

 Mauna Loa in the Sandwich Islands or as a geyser tube in Iceland. 



ORIGIN OF THE CENTRAL RANGE 



The theory of a subsequent origin for the interior range is the only 

 acceptable one. The caldera may have originated over the intersec- 

 tion of 2 fissures, one running north and south parallel to the long 

 axis of Kiushiu determining the long axis, and one at right angles 

 determining the shorter axis and affording a line of activity along 

 which a ridge of cones was later built. 



ORIGIN OF THE BARRANCO 



The single opening in the wall has been described as immediately 

 opposite the western end of the central range. It may be that its 

 origin is due to the line of weakness supposed to exist along the 

 diameter occupied by this range, and that it was contemporaneous in 

 origin with the caldera. Or it is possible that forces aided in the 

 destruction of this point in the wall at the time that great eruptions 

 were taking place along the central line and the new range was being 

 built. 



Another supposition is that it is a truncated canyon of the former 

 Mount Aso, which has cut back and tapped the floor since the caldera 

 was formed. There is not much evidence that the flanks of the former 

 Mount Aso had become deeply dissected during the interval between 

 its construction and its disappearance, and there is no other canyon 

 making a comparably deep gap in the wall. Therefore the outlet 



I Popular Science Review, New Series, Vol. IV, No. 16, October, 1880. 



