PITCHSTONE, in petrology, a glassy igneous rock having a resinous lustre and breaking with a hollow or conchoidal frac ture (Ger. Pechstein, from its resemblance to pitch). It differs from obsidian principally in its rather dull lustre, for obsidian is bright and vitreous in appearance ; pitchstones also contain water in combination amounting to from 5 to o% of their weight or o to 20% 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 of Devonian age are known (e.g., the glassy dacite of the Tay Bridge in Scotland, and the andesite-pitch stones of the Cheviots), 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 are very acid, 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 Meis sen in Saxony. These rocks are brown or dark green in colour, very frequently perlitic, and show progressive devitrification starting from cracks and joints and spreading inwards through the mass. For a long time the pitchstone dikes of Arran, Scot
land have been famous for the great beauty and variety of skele ton crystals they contain ; they are dull green in hand specimens; some of them contain phenocrysts of felspar, augite, etc., others do not, but in all there is great abundance of branching feathery crystalline growths in the ground mass, minute crystalline rods being built together in aggregates which of ten recall frost patterns. It is supposed that they consist of hornblende. In addition to these larger growths there are many small microlites scattered through the glass, also hair-like trichites, and fine rounded glob ulites. When phenocrysts are present the small crystals are planted on their surfaces like grass on a turf-covered wall. These pitch stones are believed to proceed from the great eruptive centres of early Tertiary times in western Scotland. Another pitchstone of the same period forms a great craggy ridge or scuir in the island of Eigg (Scotland).
In chemical composition this latter rock resembles the trachytes rather than the rhyolites ; in Eigg and Skye there are many dikes of pitchstone, mostly of intermediate rather than of acid character, all connected with the great eruptive activity which characterized that region in early Tertiary times.
The following analyses give the chemical composition of a few well-known pitchstones bishop (loth), formed the nuclei of the town ; and the donjon built in the loth century for Heloise, lady of Pithiviers, was one of the finest of the period.