STUDY OF STRUCTURE OF FULGURITES 685 



The white material, when flaked off and mounted in Canada 

 balsam, reveals a limpid glass, dispersed with very few and 

 minute straight microlites and very rarely rhombic plates ; the 

 latter display rather bright interference-colors between crossed 

 nicols and parallel extinction.' Where thinnest, the glass 

 remains isotrope and dark, while the thicker portion, generally 

 toward the center of the flake, glows with the greenish-white of 

 the first order of Newton's colors ; in places the color reaches 

 the sky-blue of the second order, exactly like a thin scale of 

 the underlying feldspar. The indications are that devitrification 

 had begun, throughout the glass, in globulites too extremely 

 minute for distinction, even under high magnifying power, but 

 whose concentrated effect in depolarization becomes visible, 

 between the crossed nicols, in the thicker parts of the flakes. 

 The flakes of brown or colored glass, however, are found to be 

 uniformly isotrope. It should be noted that, as the greater 

 thickness of these flakes approaches or exceeds 0.5""^, it is 

 probable that the presence of crystallites might hardly have 

 been recognized in a ground section of the ordinary thinness, 

 0.02 to 0.05"^™. In such an investigation plane surfaces are not 

 indispensable, and the examination of splinters of a glass may 

 serve an important office. Evidences of incipient devitrification, 

 in fulgurites and other glasses, have possibly escaped detection 

 by observers relying entirely on examination of thin sections. 

 The facts above described lead to the following conclusions : 

 I. We have here to do with different conditions from those 



dark globules were dispersed: "the fused surface of each crystal solidified almost 

 exactly hi situ, except where sputtering of the molten matter was caused" (RuTLEY, 

 Quar. Jour. Geo!. Soc. (1885), pp. 152-156). The inclosed globules in the slag- 

 saturated gneiss of Tuckahoe may represent a similar tendency to isolation. 



'The homogeneity of glass which apparently prevailed in all fulgurites examined 

 by the early observers led naturally to the statement in 1884 that the absence of crys- 

 tallites '"mav be used as a means of distinguishing fulgurite from other natural 

 glasses" (J. S. Diller, Ai/i. Jour. Sci., Vol. XXVIII (1884), pp. 252-258). In 1889, 

 Rutley made the first and hitherto the only record of the occurrence of devitnfica- 

 toin in a vesicular fulgurite-glass on glaucophane-schist, at Monte Viso, Cottian Alps. 

 The crystalline forms consisted of globulites, margarites, and longulites, and even 

 symmetrical microliths {Quar. Jour. Geol. Soc, Vol. XLV (1889), pp. 60-66). 



