See also:petrology, the name given to rocks which have been fused on the
See also:surface by
See also: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, 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 inch, and it may be colourless,
See also:white or yellow . When examined under the microscope, it usually shows no
See also:crystallization, and contains minute bubbles due to the expansion of 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 vertical tubes which may be nearly
See also:half an inch in diameter . Generally they are elliptical in
See also:cross section, or flattened by the pressure exerted by the surrounding 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 long axes of which are radial . Minerals like
See also:felspar and
See also:mica are fused more readily than
See also:quartz, but 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 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 .
FULGINIAE (mod. Foligno)
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