THE GKEAT KRUniON OF SAKURA-JLMA ]N 1914. 189 



with contraction- cracks and finely textured on the periphery, while 

 tlie inner part is coarse as in bread-crust bombs. Original shapes 

 are greatly modified by frictional erosion while flying through the 

 air. The size varies fi'om 2 to 5 cm., averaging 3 cm. There are 

 2 types in regard to colors. 



a) White Scum (PL XX. Fig. G). — This type is macroscopically 

 a white, friable and saccharoidal mass, which under the microscope 

 is speckled with polarizing particles of feldspar of 0.03 to 0.04 mm. 

 in diameter, enclosed by elongated pores in colorless glass- film. 

 Sometimes the feldspar particles form globular aggregates of 0.09 

 to 0.10 mm., radially fringed with elongated pores {0.07 mm. in 

 longer diameters) of glassy membrane. The feldspar polarizes with 

 brown to bluish gray of the first order. The shghtly brownish 

 color of the scum is due to the yellowish iron globules attached 

 inside the wall of the pores. 



The coarser variety is purer in color and principally made up 

 of glass-film, enclosing larger ellipsoidal pores (0.12 mm.). A few 

 hypersthene^^ and plagioclase crystals are present, being fringed 

 with radially arranged and elongated pores, which are probably due 

 to the liberation of heat during their crystallization. 



ß) Gray Scum (PI. XX. Fig. 7). — Although macroscopically 

 this type, of which the large bulk of the scums consists, difïërs 

 from the preceding in color, being ashy or bluish gray, the mic- 

 roscopic appearance is the same in both, except the thin dispersal 

 of finer particles of feldspar and the presence of aggregates of 

 magnetite globules, and of very minute and round vacuoles (0.02 to 

 0.03 mm.). The vacuoles are responsible for producing that grayish 



1) Sometimes it occurs in very fine euiiedral needles, having an appearance of brownish 

 nitile -nith totally reflecteil dark prismatic margin. Optically positive along prismatic axis, as 

 in Tutile. 



