THE GREAT ERUPTION OF SA.KURA-JIMA IN 1914. 179 



blackish, and the nature of the groundmass is responsible for 

 various shades, being entirely dependent upon the presence and 

 quantity of magnetite grains and globulites in the base. The dark 

 gray arises from the presence of globulite, and the intensity of 

 darkness is caused by the quantity of magnetite, just as in synthetic 

 gems the intensity of colors of a substance depends upon the large 

 size of the pigment. The bluish tinge indicates the presence of 

 light violet-brown glass ; the globulitic glass is never colored. 



What is stated above, chiefly refers to the main mass of the 

 western lavas. There are, however, accidental varieties of which 

 the writer is able to differentiate tico types. Both are found either 

 at the lateral margin or on the terminal lava front, and character- 

 istically free fi'om olivine, owing probably to the fact that they are 

 the salic precursors of lava effusions, and that the olivine sank 

 down in the main magma by gravitative differentiation. 



The a type (PL XIX. Figs. 3-4) is outwardly characterized in 

 being of grayish color with various shades, and of dominantly 

 feldspathic nature. The groundmass is the pilotaxitic feltwork of 

 feldspar microlites with a smaller amount of augite, which is equal 

 in size with the former. The glassy base is colorless with much 

 or no globulite. Microscopic flecks are the spots where globulites 

 are specially accumulated. 



The ß type (PI. XIX. Fig. 5) is a brown obsidian in the petro- 

 gi'apliic sense. This liyaloandesite is a variegated, dappled and 

 streaked glass, variously kneaded in wavy bands marked with 

 series of axiolites and flattened air -pores. Dark flecks are seen as 

 in the a type. 



All the historic lavas of Sakura-jima, including the most 

 recent ones, have the same appearance, structure and raineralogical 



