STUDY OF STRUCTURE OF FULGURITES 679 



this border very irregular milky threads descend into the glass 

 in a confused network, swarming with the most minute air- 

 bubbles, approaching i or 2yu. in diameter. In this vicinity, also, 

 fibrous threads or streaks of glass occur, which display, between 

 crossed nicols, feeble colors of aggregate polarization. 



On reception of this fulgurite from Mr. Walker, he stated: 

 " I have noticed in several places on the outside of the tube 

 . . . . some very fine threads of fused quartz, like delicate 

 spider-webs, connecting the sand-grains, as if the partly fused 

 grains had been forced apart while still soft." This new feature 

 in fulgurites I can confirm. The glass threads are colorless, 

 brownish, or sometimes black ; generally smooth and glistening, 

 but occasionally roughened; more or less curved; passing from 

 grain to grain of the adhering sand, but sometimes projecting as 

 if broken; mostly 0.4 to i.i™™ in length, and 0.0 15 to 0.04"™ 

 in width. ^ In the glass between neighboring sand-grains many 

 minute round holes are also perceptible "" under a low magnify- 

 ing power, apparently produced by exploded bubbles. In sev- 

 eral cases, in the glass behind a sand-grain, there is a limpid 

 band with faintly marked outline (best shown in photomicro- 

 graph. Fig. 2), the latter corresponding to the back contour of 

 the sand-grain. This must mark its original position before it 

 was jerked outward to a distance of half its diameter and the 

 space behind filled in at once with clear glass, distinct because 

 free from bubbles. To the force and extent of this outward 

 jerk, in some instances, the glass threads doubtless owe their 

 formation. 



Between crossed nicols the thin section at first appears dark, 

 like a homogeneous glass, with the exception of the ring of 

 highly refracting sand-grains outside and occasional gleams of 

 reflection from scattered air-vesicles. In ordinary light, under 



'A coating of sublimed silica is often found upon the carborundum crystals 

 manufactured in the electric furnace at Niagara Falls. The interlaced threads of 

 quartz of which it is composed much exceed the dimensions of the natural threads 

 above described. They differ also in the variation of diameter in a thread and in the 

 common occurrence of a peculiar beading along many threads. 



^ Merrill, loc. cit., p. 84. 



