14G DR 'GETKIE ON THE HISTORY OF VOLCANIC ACTION 



colour from a deep jet-black or raven-black to a pale bottle-green, and in lustre from an 

 almost glassy obsidian-like to a dull resinous aspect. Occasionally it assumes a dull 

 felsitic texture, owing to devitrification, and also a finely splierulitic structure. Some 

 varieties appear to the naked eye to be perfectly homogeneous, others become porphyritic 

 by the appearance of abundant sanidine crystals. 



The microscopic structure of the British pitchstones has only been partially worked 

 out. The beautiful feathery microlites of the Arran dykes, first made known by David 

 Forbes, and subsequently described by Zirkel, Allport, and others, are well known objects 

 to geological collectors. But no one has yet attempted to investigate the group as a 

 whole. I have placed my tolerably large collection of specimens and their slides in the 

 hands of Dr Hatch, from whom we may expect before long a memoir on this interesting 

 and still little known group of rocks. In the meantime, he has furnished me with 

 some preliminary notes on the slides, from which I make the following generalised 

 summary. 



At the one end of the pitchstone group we have a nearly pure glass, with no microlites, 

 and only a few scattered crystals of sanidine, quartz, augite, or magnetite. The glass in 

 thin slices is almost colourless, but generally inclines to yellow, sometimes to dark-grey. 

 Some varieties of the rock are crowded with microlites, in others these bodies are 

 gathered into groups, the glass between which is nearly free from them. Among the 

 minerals that have been observed in this microlitic form are sanidine, augite, hornblende 

 (forming the beautiful green feathery or fern-like aggregates in the Arran pitchstones), 

 and magnetite. Sometimes the rudimentary forms appear as globulites or as belonites, 

 but more commonly as dark trichites. Among the more definite mineral forms are 

 grains of sanidine, quartz, and augite. The porphyritic crystals are chiefly sanidine, 

 augite, and magnetite, but plagioclase occasionally occurs. The development of spheru- 

 lites is well seen in a few of the slides, and occasionally perlitic structure makes its 

 appearance. At the Scuir of Eigg, bands of a dull felsitic pitchstone occur, which under 

 the microscope show that the glass has been so devitrified as to assume a cryptocrystalline 

 structure. 



Quartz- Trachyte. — This rock has not yet been noticed in any other district than in 

 Antrim, where it rises in occasional bosses among the plateau- basalts. It is best exposed 

 at the Tardree and Carnearny Hills, where it has long been quarried. Its petrographical 

 characters were fully described by Von Lasaulx, who found the rock to be a typical 

 quartz-trachyte rich in tridymite, and containing large crystals of glassy sanidine, isolated 

 narrow laths of plagioclase (probaby andesine), grains of smoky-grey quartz, partly 

 bounded by di-hexahedral faces, and a few scattered flakes of a dark-coloured mica. The 

 ground mass is microgranitic, and under a high power is resolvable into a confused 

 aggregate of minute microlites of felspar, with interstitial quartz-granules.* 



* Tschermak's Min, und Pet. Mittheil, 1878, p. 412. 



