6/8 A. A. JULIEN 



observation, in addition to fulgurites I, II, and IV. Their 

 exclusive association, therefore, with devitrification may be a 

 consequence of more sluggish movement and imperfect recovery 

 from distortion within the more viscous crystallite-laden glass. 



Another feature, quite distinct in the vesicles of larger size, 

 is the separation of each from the surrounding glass by a limpid 

 pellicle, never exceeding o.2/ti in thickness. This appears both 

 in cross-section, with sharply differentiated outlines, and also in 

 jagged remnants around the margin of an emptied vesicle, on 

 surfaces of the thin section, like the edges of a broken eggshell. 

 It may be considered a glass coating, suddenly chilled and 

 condensed in contact with the bubble, at first consolidation of 

 the tube-wall. 



Around the outer margin of the wall occurs, as usual in 

 sand-fulgurites, a continuous row of adhering sand-grains (Fig. 

 I ), semi-fused and to that extent rendered white and opaque. 

 These grains are partly rounded and from 0.2 to 0.6™"^ across. 

 All were successively examined around the thin cross-section by 

 the usual optical methods, those of feldspar being generally 

 recognizable by traces of cleavage, cloudy alteration, oblique 

 extinction, lower interference-colors than those of quartz, and 

 more or less complete biaxial interference-figures, whose nega- 

 tive character could often be distinguished. Out of 35 grains 

 23 were identified as orthoclase, the remainder as quartz. In 

 the more angular grains wavy extinction testified to remaining 

 mechanical stress. The indications are that the original sand 

 was very fine and highly feldspathic, free from mica and with 

 few ferruginous particles. 



While the outer extremity of a sand-grain, so attached to 

 the glass wall, often retains its translucency and color entirely 

 uninjured by the electric discharge, this is sharply divided from 

 an altered milk-white inner portion, in which the change con- 

 sists mainly, with quartz, in very minute fracturing and conse- 

 quent opacity. This passes, with feldspar, into a translucent 

 border, next the glass, in which minute needles or crystallites 

 abound, suggesting immediate devitrification after fusion. From 



