Fulgurites

Fulgurites (Lat. fulgur, 'lightning'), tubes due to the action of lightning. They have been most frequently observed in loose sandhills, but have often been detected also in more compact rock. They are formed by the actual fusion of the materials through which the lightning passes. The internal surface of the tubes met with in sandhills is completely vitrified, glossy, and smooth—the thickness of the wall varying from $\backslash frac\{1\}\{3\}$th to $\backslash frac\{1\}\{5\}$th of an inch, while the diameter of the tubes ranges up to 2½ inches. They usually, but not always, descend vertically from the surface, sometimes dividing and subdividing, and rapidly narrowing downwards till they disappear. Fulgurites have often been detected on mountain-tops. In some cases the rocks attacked by lightning have the appearance of being covered with a black scoriaceous plaster, which looks as if it had 'run' or dripped. In other cases the rocks are described as being drilled—the holes produced by the lightning being lined internally with dark glassy substance. Fulgurites were first observed in 1711 by the pastor Herman, at Massel, in Silesia, and have since been found in many places; but their origin was first pointed out by Dr Hentzen in 1805.