692 A. A. J U LI EN 



* 

 long retention of the image of the flash upon the retina, its 

 duration is rarely likely to have much exceeded the above 

 quoted minute interval. 



The results of the electric action are of two classes, super- 

 ficial and internal. 



The superficial consist, in succession, of fracture of outer 

 pellicle of exposed minerals : fusion in situ of the surface of 

 more fusible grains : and development of bubbles in the enamel 

 of molten glass from expansion of moisture and air, with fre- 

 quent explosions and spattering of glass globules. 



The internal results comprise the penetration of preexisting 

 cavities of the rock, and often the actual piercing of surface of 

 outcrop or blocks, if softened and disintegrated by weathering, 

 by lightning-tubes of symmetrical cross-section, as well as pas- 

 sage downward through fissures and joints. 



This has been accompanied by fracture and fusion of pel- 

 licles over the surfaces thus traversed, especially of the more 

 fusible minerals and groundmass ; development of air- and vapor- 

 bubbles in the molten coating; and sometimes, at least, a suc- 

 cession of waves, sweeping downward fused glass and rock-dust, 

 in a series of alternating films, the oldest nearest the rock-wall 

 and subjected to frequent dislocation. 



At the moment of cessation of the electric current, there 

 have followed reaction from the intense pressure, expansion of 

 vesicles, and regurgitation of glass upon the surface of the rock 

 from some filled lightning tubes ; ^ explosion and disappearance 

 of bubbles nearest the surface, distortion of many by inward 

 relief of pressure, or general recovery of spherical form ; and 

 partial devitrification of the glass, occasionally complete, where 

 certain favorable conditions have prevailed— probably the pres- 

 ence of moist air in intermixed bubbles, a supersaturated neutral 

 glass, and an over-heated or slowly-cooling contiguous sheet of 

 rock. 



The instantaneous regeneration of feldspar only, in the form 



' First recorded by Humboldt, in rock-fulgurite on volcano of Nevada de 

 Toluca, Mexico {Ann. d. Ch. et d. Phys, Vol. XIX (1821), p. 299.) 



