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F, E. WRIGHT OBSIDIAN FROM ICELAND 



lines of the pyramids of the cube, we then aHow crystallization to proceed 

 continuously with concomitant evolution of volatile components, which 

 tend to force the rigid walls still farther apart, we oljtain the present 

 forms. Evidence that this has been the process of development is not 

 lacking. 



( 1 ) The radius of curvature of the outer wall of the right side of the 

 lithophysa on the right in figure 6 is variable. It is least in the center 

 of the segment and becomes increasingly greater as the margin of the 

 segment is approached; near the margin the curve of the outei- wall 

 shows a flexure. The edges of the lithophysal cul)e are veiy thin ami 

 onlv a thin film of crystallized material has been foi-nied next to the 



FiULUK y. — Biuf/i uiniitatir Ueprvsentation 

 of Cube 'built vp of nix Pyramids 



The apices of the pyi'amids must meet 

 at the center and their ijases are the sides 

 of the cube. 



FiGURK 10. — ElUiJ-sdid-like litlmiiliijsul Cuc- 

 ity, ivith central Girdle of crystallized 

 Material 



Specimen n,s4:;i. Xatural size 



glass, thus indicating that crystallization was active only a relatively 

 short time at that point. The ratio of the thickness of the crv'stallized 

 shell at the center of a segment to that of the crystallized lilm at its 

 edge is of the order of magnitude of 50 to 1. 



(2) On one side of specimen 88431 a cavity, 9 mm. in diameter, oc- 

 curs, out of which the crystallized material has fallen, except for a single 

 equatorial groove (see figure 10 ) . In this case it appears that the spheru- 

 lite was not broken into halves until it had grown to an appreciable size, 

 and that then it was forced apart along a single plane, thus elongating the 

 sphere into an ellipsoid-like figure with a central girdle. 



(3) A difference in age between the center and the margin of the 

 exposed faces of the lithophysa is clearly indicated by tiie change in 

 granularity of the recrystallized material. Beginning at the center (fig- 



