PITCHSTONE (

See also:

• GERMAN
• GERMAN, DUTCH AND SCANDINAVIAN

German Pechstein, from its resemblance to

See also:

• PITCH
• PITCH, MUSICAL

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

For a

long time the pitchstone dikes of Arran in Scotland have been famous among geologists for the great beauty and variety of skeleton crystals they contain . These pitchstones are dull green in hand specimens . Some of them contain phenocrysts of felspar, 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 fir trees or the fronds or ferns, minute crystalline rods being built together in aggregates which often recall the frost patterns on a window-pane . It is sup-posed that the mineral they consist of is hornblende . In addition to these larger growths there are many small microlites scattered through the glass, also hair-like trichites, and fine rounded globulites . When phenocrysts are present the small crystals are planted on their surfaces like grass growing from a turf-covered wall . These pitch-stones are believed to proceed from the great eruptive centres which were active in western Scotland in early Tertiary times . Another pitchstone of the same period forms a great craggy ridge or scuir in the island of Eigg (Scotland) . At one time regarded as a lava flow occupying an old stream channel it has recently been described as an intrusive 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 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

rich brown colour in thin section with well 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 character, while the third is richer in alkalis and contains less silica; it belongs more naturally to the intermediate rocks (or trachytes.) (J . S .