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PITCHSTONE (German Pechstein, from it...

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Originally appearing in Volume V21, Page 665 of the 1911 Encyclopedia Britannica.
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PITCHSTONE (See also:German Pechstein, from its resemblance to See also:pitch) , in See also:petrology, a glassy igneous See also:rock having a resinous lustre and breaking with a hollow or conchoidal fracture. It differs from See also:obsidian principally in its rather dull lustre, for obsidian is See also:bright and vitreous in See also:appearance; all pitchstones also contain a considerable quantity of See also:water in See also:combination amounting ' to from 5 to 10% of their See also:weight or 10 to 2o% of their See also:volume. The See also:majority of the rocks of this class occur as intrusive dikes or See also:veins; they are glassy forms of See also:quartz See also:porphyry and other See also:dike rocks. Their dull lustre may be connected with the See also:great abundance of See also:minute crystallites and microlites they nearly always contain. These are visible only in microscopic sections, and their varied shapes make pitchstones very interesting to the microscopist. Although pitchstones are known which are of Devonian See also:age (e.g. the glassy See also:dacite of the See also:Tay See also:Bridge in See also:Fife, See also:Scotland, and the See also:andesite-pitchstones of the Cheviot Hills), most of them are See also:Tertiary or See also:recent, as like all natural glasses they tend to crystallize or become devitrified in course of See also:time. In some of the older pitchstones the greater See also:part of the See also:mass is changed to a dull felsitic substance, while only nodules or kernels of unaltered See also:glass remain. Some pitchstones are very See also:acid rocks, containing 70 to 75 % of See also:silica, and have See also:close chemical See also:affinities to granites and rhyolites. Others contain more alkalis and less silica, being apparently vitreous types of See also:trachyte or keratophyre; others have the See also:composition of dacite and andesite, but the See also:black basaltic glasses are not usually classified among the pitchstones. Very well known rocks of this See also:group occur at See also:Chemnitz and See also:Meissen in See also:Saxony. They are See also:brown or dark See also:green, very often perlitic (see PETROLOGY, See also:Plate I., fig. 5), and show progressive devitrification starting from cracks and See also:joints and spreading inwards through the mass.

For a See also:

long time the pitchstone dikes of See also:Arran in Scotland have been famous among geologists for the great beauty and variety of See also:skeleton crystals they contain. These pitchstones are dull green in See also:hand specimens. Some of them contain phenocrysts of See also:felspar, See also:augite, &c. ; others do not, but in all there is great abundance of branching feathery crystalline growths in the ground mass: they resemble the branches of See also:fir trees or the fronds or ferns, minute crystalline rods being built together in aggregates which often recall the See also:frost patterns on a window-See also:pane. It is sup-posed that the See also:mineral they consist of is See also:hornblende. In addition to these larger growths there are many small microlites scattered through the glass, also See also:hair-like trichites, and See also:fine rounded globulites. When phenocrysts are See also:present the small crystals are planted on their surfaces like grass growing from a See also:turf-covered See also:wall. These pitch-stones are believed to proceed from the great eruptive centres which were active in western Scotland in See also:early Tertiary times. Another pitchstone of the same See also:period forms a great craggy See also:ridge or scuir in the See also:island of Eigg (Scotland). At one time regarded as a See also:lava flow occupying an old stream channel it has recently been described as an intrusive See also:sheet. It is from 200 to 300 ft. thick. The rock is a dark, nearly black, pitchstone-porphyry, with glancing idiomorphic crystals of felspar in a vitreous See also:base.

It contains no quartz; the felspars are anorthoclase, and with them there are numerous crystals of green augite. The ground mass contains small crystallites of felspar, and is of a See also:

rich brown See also:colour in thin See also:section with well See also:developed perlitic structure (see PETROLOGY, Plate II., fig. I). In The first two of these contain much water for rocks the ingredients of which are but little decomposed. They are of acid or rhyolitic See also:character, while the third is richer in alkalis and contains less silica; it belongs more naturally to the intermediate rocks (or trachytes.) (J. S.

End of Article: PITCHSTONE (German Pechstein, from its resemblance to pitch)

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