37° 
THE TERTIARY VOLCANOES 
BOOK VIII 
tures, varying from microscopic minuteness up to large round or egg-shaped 
balls nearly two inches in diameter, and often distributed in lines along 
those of flow-structure. They likewise exhibit a frequent development of 
micropegmatite. No line indeed can be drawn between these felsites and 
the granitoid varieties, for the same characteristic grauophyric intergrowtli 
of felspar and quartz runs through them all. 
Pitch-stone . — This name is applied to the glassy varieties apart from 
their chemical composition, and specially denotes the possession of a vitre- 
ous structure. Some of the rocks to which it has been applied are prob- 
ably glassy varieties of andesite, others are dacites, while some may be as 
acid as the most acid felsites and granophyres. The pitchstones are found 
in veins or dykes which traverse different geological formations up to and 
including the great granophyre bosses of the Inner Hebrides, They vary in 
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 they assume a felsitic texture, owing to devitrification, and 
also a finely spherulitic structure. Some varieties appear to the naked eye 
to be perfectly homogeneous, others become porpliyritic by the appearance 
of abundant sanidine crystals. 
The microscopic structure of the British pitchstones has not yet been 
fully 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. Dr. Hatch, 
in whose hands I placed my tolerably large collection of specimens and their 
thin slides, furnished me with some preliminary notes on the slides, from 
which the following generalized summary is compiled. 
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 magetite. 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 aggre- 
gates in the Arran pitchstones, Fig. 3) and magnetite. Sometimes the rudi- 
mentary 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 porpliyritic crystals are chiefly sanidine, augite and 
magnetite, but plagioclase occasionally occurs. The development of spherulites 
is well seen in a few of the slides, and occasionally perlitic structure makes 
its appearance. 
The interesting rhyolitic areas of Antrim include several varieties of 
pitchstone. One of these is described by Professor Cole as “ a glassy 
pyroxene-rhyolite, on the verge of the rhyolitic andesites.” Another is a 
blue-black porpliyritic obsidian. 1 
1 Scientif. Trails. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77. 
