156 S. SÈKIYA AND Y. KIKUCHI 



of the water which has accumulated within the crater-basin from the 

 condensing vapours, appears to show that corrosive gases have been 

 active within the volcano, decomposing and altering the rocks. These 

 acid gases have no doubt affected the rocks, by combining with bases 

 to form soluble salts, which are now dissolved in the waters of the 

 new lake within the crater, leaving as residue the white coloured 

 sintery material, which consists almost wholly of silica. 



Microscopically examined this white rock is seen to have been 

 entirely altered in substance, but the original form of the por- 

 phyritic crystals is more or less preserved. A completely isotropic 

 colourless substance is found filling up the spaces of all the crystal- 

 sections, which nevertheless exhibit the characteristics of the original 

 minerals. The decomposition of the andesitic rock by the action of 

 acid gases had doubtless the effect of decomposing the mineral 

 matters ; the silica first separating out in a gelatinous form, 

 and then consolidating to a porodine-amorphous state. Thus the 

 isotropic substance mentioned above seems to be silica in the form 

 of opal. 



The tissures of these crystals are often filled with crystals of 

 sulphur which may frequently be detected by the naked eye as well 

 as under the microscope. Under crossed niçois it may be recognised 

 by the fact that certain faint yellow coloured portions within the 

 section exhibit vivid interference-colours, while the general ground is 

 entirely dark. The crystals of plagioclase sometimes retain their 

 original substance, which the pyroxene crystals never do. In the 

 latter case it is interesting to observe that the alteration of the original 

 substance into a fibrous viridite-like product had began before it was 

 completely decomposed into its present state, as we can still detect in 

 it the characteristic trace of this alteration in the manner already 

 described. 



