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FULGURITE (from Lat. fulgur, lightning)

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Originally appearing in Volume V11, Page 293 of the 1911 Encyclopedia Britannica.
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FULGURITE (from See also:Lat. fulgur, See also:lightning) , in See also:petrology, the name given to rocks which have been fused on the See also:surface by lightning, and to the characteristic holes in rocks formed by the same agency. When lightning strikes the naked surfaces of rocks, the sudden rise of temperature may produce a certain amount of See also:fusion, especially when the rocks are dry and the See also:electricity is not readily conducted away. Instances of this have been observed on See also:Ararat and on several mountains in the See also:Alps, See also:Pyrenees, &c. A thin glassy crust, resembling a coat of See also:varnish, is formed; its thickness is usually not more than one-eighth of an See also:inch, and it may be colourless, See also:white or yellow. When examined under the See also:microscope, it usually shows no See also:crystallization, and contains See also:minute bubbles due to the expansion of See also:air or other gases in the fused pellicle. Occasionally small microliths may appear, but this is uncommon because so thin a film would cool with extreme rapidity. The minerals of the See also:rock beneath are in some cases partly fused, but the more refractory often appear quite unaffected. The See also:glass has arisen from the melting of the most fusible ingredients alone. Another type of fulgurite is commonest in dry sands and takes the shape of See also:vertical tubes which may be nearly See also:half an inch in See also:diameter. Generally they are elliptical in See also:cross See also:section, or flattened by the pressure exerted by the surrounding See also:sand on the fulgurite at a See also: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. Tubular perforations in hard rocks have been noted also, but these are See also:short and probably follow See also:original cracks.

The glassy material contains grains of sand and many small See also:

round or elliptical cavities, the See also:long axes of which are radial. Minerals like See also:felspar and See also:mica are fused more readily than See also:quartz, but See also:analysis shows that some fulgurite glasses are very See also:rich in See also:silica, which perhaps was dissolved in the glass rather than simply fused. The central cavity of the See also:tube and the bubbles in its walls point to the expansion of the gases (air, See also:water, &c.) in the sand by sudden and extreme See also:heating. Very See also:fine threads of glass project from the surface of the tube as if fused droplets had been projected outwards with considerable force. Where the quartz grains have been greatly heated but not melted they become white and semi-opaque, but where they are in contact with the glass they usually show partial See also:solution. Occasionally crystallization has begun before the glass solidified, and small microliths, the nature of which is undeterminable, occur in streams and wisps in the clear hyaline See also:matrix. (J. S.

End of Article: FULGURITE (from Lat. fulgur, lightning)

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