FULGURITES OR LIGHTNING-HOLES. 537 



of the flash. Indeed, these authorities considered the presence of water 

 in the sand as essential to the formation of the tube. Wichmann does 

 not fully acquiesce in the view that the wings are produced by the col- 

 lapsing of a portion of the tube-walls, but considers them as original 

 formations. 



That they are not due simply to collapsing is the opinion of the 

 present writer. In the article quoted he there expresses the opinion 

 that the irregular form of the tube near the surface is due to the ex- 

 ceeding energetic action of the current during this part of its course, 

 and the lack of homogeneity in the conducting material. At depths 

 of a few feet below the surface, where the force of the current had 

 become to some extent reduced, and the sand was more compact and 

 homogeneous, the tube was found more nearly cylindrical.* It is very 

 probable that steam may have been instrumental in producing the 

 bulb-like enlargements so commonly found, but it can scarcely be con- 

 sidered as essential to the formation of the tube itself. 



We have next to notice the fulgurites produced upon solid rock. 

 These, as can readily be imagined, differ from those produced in the 

 sand, in being of but slight depth, and frequently existing merely as 

 a thin, glassy coating on the surface. 



G. Rose f describes fulgurites of this nature as occurring in abun- 

 dance upon the summit of Little Ararat, in Armenia. The rock is an 

 andesite, somewhat soft and porous, and it is stated that blocks a foot 

 long can be obtained, perforated in all directions by the irregular 

 tubes, from three to five centimetres in diameter, which are filled 

 with a bottle-green glass, formed from the fused rock. 



A small specimen of this rock, deposited in the National Museum, \ 

 has much the appearance of a rock bored by the teredo, the holes in 

 which have subsequently been filled by the green glass. It is stated 

 by this writer that, in fulgurites collected by Humboldt from the 

 Punta del Fraile, in Mexico, the fused mass of the walls had over- 

 flowed the tubes upon the surrounding surfaces. Saussure describes 

 fulgurites as occurring also in the hornblendic schists of Mont 

 Blanc, and Ramond mentions similar occurrences on Monts Perdu 

 and Pic du Midi in the Pyrenees, as well as upon Mont Auvergne 

 in France.* 



Wichmann || examined the Ararat fulgurite glass with the micro- 

 scope, and found it to agree closely with that of those formed in loose 

 sand, being completely amorphous without trace of microlites, and in 

 containing numerous steam cavities. 



* He also shows that, even in so homogeneous a material as sheet-copper, the hole 

 produced by electric fusion is not in all cases circular in outline, but is often quite irregu- 

 lar, closely resembling a cross-section of fulgurite tube. (See Fig. 4 of plate.) 



f " Zeit. der deutsch. geol. Gesell," voL xxv, p. 112. 

 % The property of Mr. J. S. Diller. 



* " Ann. de Chim. et Physique," Bd. xix, p. 155. 



\ "Zeit. der deutsch. geol. Gesell.," vol. xxxv, p. 858. 



