Page:Popular Science Monthly Volume 30.djvu/557

Rh 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 collapsing 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 exceeding 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 considered 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 describes fulgurites of this nature as occurring in abundance 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 overflowed 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 microscope, 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.