686 A. A. J U LI EN 



which attended the sand-fulgurite I, viz., the lessening force of a 

 divided electric current, passing over the surface of a compact 

 rock-mass. The effect of the electric action on this solid sur- 

 face of gneiss has been more diffuse, feeble, and superficial than 

 in the sand-fulgurite, and, in this case, confined to the feldspar- 



2. The fulgurite crust has been produced almost entirely by 

 fusion of surfaces of feldspar-grains rather than of quartz, and 

 in small degree by that of the iron-containing minerals. The 

 property of fusibility, rather than that of imperfect conduction, 

 has determined the amount of attack on each grain. Each kind 

 of glass, colorless and colored, is sharply confined to the min- 

 eral surfaces from whose fusion it originated, with little tendency 

 to intermixture. 



3. The bubbles throughout the fulgurite-crust owe their for- 

 mation chiefly to expansion of air, and in part doubtless of 

 steam, derived from moisture in the weathered surface of the 

 rock. Their sputtering explosion has probably produced the 

 tiny fibers over the blebby surface. 



4. The surprising partial devitrification, which has instanta- 

 neously followed fusion throughout the delicate white pellicle, 

 has been evidently facilitated by the supersaturated feldspar 

 solution of which the molten glass almost entirely consisted, 

 and by its consequent unstable molecular condition. 



III. Fulgurite in augite-andesite, summit of Lesser Ararat, 

 Armenia. The specimen was one of those collected by Dr. E. 

 O. Hovey, of the American Museum of Natural History, New 

 York. 



The fulgurite material from this peak has received repeated 

 study in the field or laboratory by successive observers. Abich'' 

 detected only pure glass in the fulgurite, but in such abundance 

 that he suggested the name " fulgurite-andesite" for the mate- 

 rial on the apex of the peak. Gustav Rose^ also came to the 

 same conclusion, and determined the difficult fusibility of the 



"- Sitz. Akad. Wiss. Wien, Vol. LX (1870), I. Abth., pp. 153-161. 

 ■^Zeit. d. D. geol. Ges., Vol. XXV (1873), pp. 112, 113. 



