THE GKEAT ERUPTION OE SAKUEA-JIMA IN 1914. 205 



posed to be attributable to high temperature (A. von Las aulx), 

 are frequently seen. Optically positive, and pleochroic only in 

 larger individuals ; c >(i = a. 



A characteristic feature is the accumulation of variously ori- 

 entated grains (Laceoix's colloidal cordierite), of which the whole 

 interior of the cordierite is built up (PL XXII. Figs. 4 and 6), and 

 such composite crystals appear macroscopically dark-blue. When 

 clear crystals are seen, minute pores are characteristically found in 

 the central part, thereby coloring allochromatically grayish- 

 b 1 u e ,^^ and the vacuoles are probably, originally, open spaces left 

 between granules, as in porcelain pores. 



Larger cordierite is habitually resolved peripherally into grains 

 (colloidal cordierite), or microcrystals rudely arranged parallel to the 

 c-axis (PI. XXII. Fig. 3). Both the external and internal granula- 

 tions seem to have risen from regressive resorption rather than 

 from imperfect growth. Where roundish clear plagioclase is en- 

 closed poikihtically within the vacuole-rich cordierite, the former is 

 fringed with isotropic substance, as if it were a reactional rim 

 (the corona of French authors), which sometimes contains gas 

 pores (PI. XXII. Fig. 2). It may arise from remelting through a 

 temporary rise of temperature. 



Glass makes up the colorless groundmass in variable quantity, 

 in which an excess of SiO., found in the analysis, is likely to be 

 contained. It is always fuU of pores. When the vacuoles sink 

 to 0.02 to 0.03 mm., the glass appears macroscopically gray on 

 account of total reflection of light ; otherwise it looks dull-white. 



As to the mode of formation of ceramicites or cordieritiferous 

 éjecta, the petrologists are not in accord in their opinions, but 



1) The writer conjectures that the characteristic hluisJi colors and pleochroism of cordierite 

 are attributable to the same cause. 



