Page:Popular Science Monthly Volume 20.djvu/676

656 metre, on which the rock appears to be veneered with a kind of gray or yellowish enamel, in which bubbles or swellings several millimetres in diameter may be distinguished. The specimen represented in Fig. 1, which is exhibited in the geological department of the museum at

 (From a specimen in the Museum; figure one fourth the natural size.00)

Paris, was found on the top of the Pic-du-Midi by MM. Baylac and Albert Tissandier. It is of special interest. The rock is of a granitoid diorite, that is, a mixture of triclinic feldspar and hornblende amphibole. The melted portion does not constitute a veneer, as in the examples previously mentioned; it is a track exactly marking the course of the electric spark and ramifying as it did. The vitrified portion extends along the natural external surface of the rock, and then plunges into a fissure, within which it disappears. In this respect the fulguration is extremely like another accident which is known under the name of fulgurites or fulminated tubes, splendid specimens of which may be seen at the museum. As the two cuts in Fig. 2 show, there are irregular tubes, the substance of which, a kind of natural glass, is the product of the solution of siliceous sands that have been struck by lightning. The tube is smooth within, but rugose on the outside, on account of the agglutination of imperfectly melted particles of sand. Fulgurites are generally ramified at their lower end. Their interior diameter varies from a millimetre to five centimetres, or two inches; and their length, which is variable, may reach ten metres, or more than thirty feet.

These curious accidents do not seem to have been remarked before 1711, when Hermann observed them in Silesia; since then all the