1911 Encyclopædia Britannica/Pitchstone

by John Smith Flett

PITCHSTONE (German Pechstein, from its resemblance to 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 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 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 supposed 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 pitchstones 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., flg. 1). In chemical composition this 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 1n early Tertiary times.

The following analyses give the chemical composition of a few well-known pitchstone:—

	SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O	K2O	H2O
I. Meissen, Saxony . . . .  II. Corriegills, Arran . . . .  III. Scuir of Eigg, Scotland.	72.42 72.07 65.81	11.26 11.26 14.01	0.75 3.24 4.43	0.28 tr. 0.89	1.35 1.53 2.01	2.86 0.61 4.15	3.80 5.61 6.08	7.64 5.45 2.70

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. F.)