5o8 ROBERT ANDERSON 



and supports a fine grove of trees. The higher portion is precipi- 

 tous and exposes best the material of which it is made. It is formed 

 of roughly bedded flows of basaltic andesite, interbedded and inter- 

 mingled with mixtures of vesicular lava, scoriae, pumice, and volcanic 

 sand. The lava predominates, and the harder layers project with 

 vertical rocky faces, while between them softer zones have weathered 

 way into debris slopes. The height of the wall is on the average 

 about 1,300 to 1,500 feet. It decreases toward the eastern side 

 owing to the gradual rise of the floor in that direction, but increases 

 at some points, as on the southwest and west sides, where mountains 

 break the regularity of the horizon line. 



The only opening in the ring wall is the barranco on the eastern 

 side through which the streams have their outlet. Cliffs of roughly 

 columnar lava form the sides of this gorge up to a height of several 

 hundred feet and are continued upward by densely wooded slopes 

 that rise over 1,500 feet above the stream bed. The span of the 

 canyon where it begins to widen out above the cliffs is less than half 

 a mile. 



Probably the next lowest point upon the rim is at a point three 

 miles north of the outlet, where the "Futaetoge" {toge means "pass"), 

 which has an elevation of about 2,500 feet above the sea, is less than 

 1,000 feet above the caldera floor. Another stream running down 

 the outer slope to the sea heads at this low point. The highest portion 

 of the watershed on the rim is on Tawara-yama and Kamuri-dake, 

 two mountains that form the wall at the western end of the southern 

 basin. They rise 2,000 to 2,700 above the floor of the caldera. The 

 former is between two and three miles south of the outlet of the 

 streams, the latter four miles or more. The summit of each and the 

 high ridge connecting them is about two miles back from the floor. 



THE CENTRAL RANGE 



The ridge or range that divides the caldera is, properly speaking, 

 Mount Aso. From the floor on the south, or the one on the north, it 

 appears as a massive rugged barrier the summit of which is roughened 

 by several dominating peaks. (See Figs. 3 and 5.) Looking up at 

 it from either basin one would never imagine the existence of a second 

 and almost identical pit with level floor and encircling outer wall at 



