514 ROBERT ANDERSON 



used for three days; in 1889 during the year of the Kumamoto 

 earthquake, which was the year following the great explosions of 

 Bandai-san in central Japan; and lastly in 1894, when the floor of 

 the modern crater was somewhat altered. 



THE OUTER SLOPES 



The vast open bowl of Aso occupies the summit of a great mound, 

 the flanks of which, sloping gently away from the top of the interior 

 wall, form a rolling hilly upland of low relief among the mountains 

 of Kiushiu. This low dome-shaped mountain taken as a whole 

 occupies at least 450 square miles. From above, the outer slope 

 appears as an inclined plateau wrinkled with knolls and ridges and 

 little valleys. All of these hillocks are of similar height, and destruc- 

 tion through erosion has not gone far. The general angle of slope 

 away from the edge of the caldera is about 3 to 5°, but it is even 

 less than this in places. On the northwest side it is very shght, because 

 the flanks of the mound, which reach so far in other directions, there 

 give place to a plateau between the rim and the not far distant vol- 

 canic mountains Kura-take and Ona-take. Within five to ten miles of 

 the rim on most sides, at an elevation above the sea about the same as 

 that of the bottom of the caldera, the outer slopes reach the foot of 

 high mountains that almost completely encircle the upland, much as 

 the caldera wall surrounds the plain formed by its floor. The sur- 

 rounding mountain barrier is not volcanic on the east and south, 

 where ancient sedimentary and igneous rocks form Sobo-san and 

 other prominent peaks. The whole volcanic mass of Aso and of the 

 mountains to the north is of the nature of a filHng within a depression 

 in the topography of the older formations. 



The surface material of the outer slopes is largely fragmental 

 debris. At a few places where it was observed exposed it appeared as 

 massive deposits of sandy material, or as soft tuff, or as agglomerate 

 composed of coarser fragments. Some of the hills many miles from 

 Aso are entirely composed of fragmental volcanic deposits of this 

 kind. Lava is frequently exposed on the western slope down to the 

 Kumamoto plain; and in the upland region about Aso, toward the 

 foot of the surrounding mountains, where dissection has advanced 

 farthest, lava is much in evidence. It is most probable that 



