FULGURITE, in petrology the name given to rocks which have been fused on the surface by lightning, and to the char acteristic holes in rocks formed by the same agency (from Lat. fulgur, lightning) . When lightning strikes the naked surfaces of rocks, the sudden rise of temperature may produce a certain amount of fusion, especially when the rocks are dry and the elec tricity is not readily conducted away. Instances of this have been observed on Ararat and on several mountains in the Alps, Pyre nees, etc. A thin glassy crust, resembling a coat of varnish, is formed ; its thickness is usually not more than one-eighth of an inch, and it may be colourless, white, yellow, brown or black. Another type of fulgurite is commonest in dry sands and takes the shape of vertical tubes which may be nearly half an inch in diameter. Generally they are elliptical in cross section, or flat tened by the pressure exerted by the surrounding sand on the fulgurite at a time when it was still very hot and plastic. These tubes are often vertical and may run downwards for several feet through the sand, branching and lessening as they descend. Tubu lar perforations in hard rocks have been noted also, but these are short and probably follow original cracks. The central cavity of the tube and the bubbles in its walls point to the expansion of the gases (air, water, etc.) in the sand by sudden and extreme heating. Very fine threads of glass project from the surface of the tube as if fused droplets had been projected outwards with consider able force. (J. S. F.)