5i6 ROBERT ANDERSON 



by the writer to its termination on the east coast. The contact of the 

 volcanic sheet and the older rocks in that direction is about twenty 

 miles from the center of Aso, beyond which the lava extends down 

 the canyon about thirty miles, almost to the sea. The width of the 

 present lava filling of the canyon is on the average two and one-half 

 to three miles, and the depth amounts certainly to several hundred 

 feet. Nearly twenty-five miles away from the center of Aso the 

 Gokase is joined by another large canyon from the north that issues 

 from mountains in the Paleozoic formation. The lava flowed up 

 this canyon for a distance of at least eight miles, filling its bottom 

 likewise with a wide, deep stream. The source of the lava in this 

 branch was the main stream that occupied the Gokase canyon. It 

 could not have come down as a tributary flow because no volcanoes 

 exist anywhere about. 



High mountains formed of Paleozoic rocks inclose the valley of 

 the Gokase-gawa, rising steeply above the fairly flat surface of the 

 lava filling. In the center of this the river has cut an abrupt square 

 canyon, and has already reached a depth of several hundred feet with- 

 out coming to the level of its old course. The depth of the channel 

 is at least three hundred feet in some places and its width is hardly 

 more. Its sides are cliffs of basalt-andesite, usually with imperfect 

 columnar structure. The columns, which average a foot or more 

 in thickness, are sometimes four-sided, sometimes five-sided, the sides 

 and angles being of irregular dimensions. They stand in the main 

 perpendicular, but locally show deflections of a few degrees from this 

 attitude. 



CHARACTER OF THE LAVA 



In the field the lava of the interior range, the ring wall and the 

 surrounding region appeared to be the same. The rock is inter- 

 mediate between andesite and basalt. That it is by no means of the 

 most basic composition is indicated by the angle of the slopes pre- 

 sented in the interior range, and by the fact that explosive eruptions, 

 with the ejection of abundant ashes and scoriae, have taken place in 

 association with effusive eruptions throughout the history of the 

 volcano. On the other hand, the recent-looking flows observable on 

 the higher portion of the central range exhibit a smooth flow structure 

 that is evidence of considerable fluidity and a prolonged state of fusion. 



