5i8 ROBERT ANDERSON 



mass contains plagioclase, brown glass, magnetite, augite, and possibly 

 a little olivine. Specimen 3 is a very vesicular fragment of hypersthene 

 andesite blown from the new crater. It is composed of phenocrysts 

 — labradorite and hypersthene in a groundmass of brown glass. Re- 

 garding these specimens Doctor Johannsen says as follows: "At 

 best the amount of olivine in any of the rocks is slight, and with its 

 absence I would name them all andesite." 



A SUPPOSED FORMER MT. ASO 



The roughly bedded strata in the walls of the big crater seem to 

 dip away on all sides at a low angle, and their incUnation is probably 

 reflected in the gentle outer slopes that form the sides of the mound 

 or cone of Aso. It seems Hkely that this mound is the basal remnant 

 of a conical volcano that once continued upward to a culminating point 

 high above the center of what is now the crater bowl. 



If such a mountain existed it is probable that its upper portion rose 

 with a gradually increasing slope into a summit cone. Judging 

 from the character of the lava and the analogy afforded by the steep 

 slopes of the present interior peaks of Aso, which seem to be con- 

 structed of materials similar to those of the wall, as well as by other 

 volcanoes of Kiushiu (see Fig. i), which are mostly built of similar 

 andesitic lava and its fragmental products, the ancient cone may 

 have risen in its upper portion even as steeply as 20° or 30°. But 

 assuming that it rose with a constant slope no greater than now in 

 places exhibited in the base, say 7°, its summit would have been over 

 7,000 feet in altitude above the sea. If it steepened above it may 

 have been 10,000 feet or much more. 



The amount of rock material that must have been removed to 

 cause the disappearance of the whole upper portion of the cone and 

 the opening of a wide bowl upon its site may be roughly estimated 

 as at least fifty-four cubic miles, counting the volume of the caldera as 

 twenty-five cubic miles — not subtracting the interior range — and the 

 volume of the overlying cone as twenty-nine cubic miles. ^ If the 

 cone rose steeply above, as it probably did, the volume must have 

 been considerably more. 



I The figures printed in the article by the writer in the Popular Science Monthly, 

 Vol. LXXI, July, 1907, are incorrect. 



