158 S. SEKIYA AND Y. KIKUCHI 



grey colour, is usually very fine grained, but .sometimes mixed with 

 coarser rock- fragments. The microscopic examination of the dust 

 shows that it is essentially made up of the same mineral components 

 as the Pyroxene-andesite already described, proving that it was deriv- 

 ed by the mechanical trituration of this rock. It consists of minute 

 particles of the microfelsitic groundmass, mixed with crystal-frag- 

 ments of Plagioclase, Sanidine, Augite, Hypersthene, Magnetite and 

 Apatite needles with a very small amount of glass. 



A specimen of the dust was brought from the town of Miliar u, 

 38 kilometres to the east of Bandai-san. It is essentially the same 

 as that which fell in the immediate neighbourhood of the volcano, 

 only that it is somewhat liner grained. In mineralogical composi- 

 tion also, it is almost exactly similar, being chiefly made up of the 

 finer particles of the rock, the crystal-fragments of plagioclase, the 

 pyroxenes, and magnetite. These ashes, being comparatively heavy, 

 do not seem to have fallen to any very great distance, thus differing 

 from the fleecy pumiceous materials produced from molten lava at the 

 time of other volcanic eruptions. 



The following chemical analysis (I.) of the ash obtained by Mr. 

 S. Otsuka of the Geological Survey, from Hikage in Biwa-sawa, when 

 compared with those of the andésite rocks already given, will show 

 how close is the agreement. 



This ash, when treated with a mixture of hydrochloric acid, mixed 

 with nitric acid in order to dissolve out the sulphur, gave a residue 

 amounting to 52*92 °/ . This, when analyzed, gave the result as in 

 column II. 



