THE ERUPTION OF BANDAI-SAN. 155 



dark, often compact, and resinous ground-mass, thus presenting a 

 marked porphyritic structure. 



Besides occurring in the form of a sheet of solid rock, it is also 

 abundant among 1 the fragments which compose the agglomerate, 

 being, in this case, often «coriaceous in appearance. Under the 

 microscope, the ground-mass is micro- crystalline, with a more or less 

 brownish coloured glass-basis. In the scoriaceous rock just referred 

 to, the hypersthene crystal is often highly pleochroic. The specific 

 gravity of the plagioclase determined with Thoulet's solution is found 

 to be little higher than that contained in the first type, being on an 

 average 2* 691. 



To this type also belongs the rock which is exposed in a rugged 

 cliff of the half destroyed Yugeta-yama, on the western edge of the 

 new crater, and the analysis of which is given above (I). In this 

 rock there is a very light brownish coloured glass-basis among the 

 microlithic ground-mass. The black colour of the rock is, however, 

 in this ease, also due to another cause, viz., to the alteration which 

 the crystals of pyroxenes have undergone. These crystals are seen 

 to have altered into a peculiar dirty green coloured fibrous substance 

 like viridite, the alteration proceeding from the fissures or from the 

 periphery and often forming a pseudomorph. The altered product, 

 diffusing itself into the surrounding ground-mass, imparts a peculiar 

 dark greenish hue to the appearance of the rock. The alteration also 

 often results in the formation of a reddish coloured iron-oxide. Some- 

 times the crevices of the rock are filled with iron-pyrites. It is 

 highly probable that these phenomena have been greatly due to the 

 action of fumaroles and hotsprings, the rock being found in close 

 proximity to the Kaminoyu spa. 



The following analysis of the white coloured rock, already referred 

 to (p. 113), found near to the steam-fissures, together with that 



