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 aggregates in the Arran pitchstones, [Fig. 3]) 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 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 porphyritic obsidian.[369]
[369] Scientif. Trans. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77.
Rhyolite (Quartz-Trachyte).—This rock has been abundantly erupted in north-east Ireland, where it rises in occasional bosses among the plateau-basalts.[370] It is best exposed at the Tardree and Carnearny Hills, where it has long been quarried. Its petrographical characters at that locality were described by Von Lasaulx as those of a typical quartz-trachyte rich in tridymite, and containing large crystals of glassy sanidine, isolated narrow laths of plagioclase (probably andesine), grains of smoky-grey quartz, partly bounded by dihexahedral faces, and a few scattered flakes of a dark-coloured mica. The groundmass is microgranitic, and under a high power is resolvable into a confused aggregate of minute microlites of felspar, with interstitial quartz-granules.[371] More recently a detailed investigation of the petrography of the Antrim rhyolites has been conducted by Professor Cole, who has called attention to their remarkable varieties of structure, ranging from perfect volcanic glass to a thoroughly lithoidal texture, and exhibiting flow, perlitic and spherulitic structures.[372]
[370] Fragments of acid rock were detected by Prof. Cole in the gravel among the Ardtun basalt of Mull, as already noticed on p. 212.
[371] Tschermak's Min. und Pet. Mittheil. 1878, p. 412.
[372] Scientif. Trans. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77. This paper gives an excellent account of the microscopical character and mineralogical and chemical compositions of these rocks.
Intrusive masses of rhyolite are also found in the Carlingford region. One of these, seen at Forkhill, is a velvet-black almost resinous rock with abundant quartz and felspar, and sometimes displaying beautiful flow-structure. It will be more particularly described in Chapter xlvii. Some of the acid dykes and sills of the Inner Hebrides are varieties of rhyolite. No undoubted example has yet been observed of a superficial rhyolite-lava, though such not improbably appeared in the interval between the lower and upper basalts of Antrim.
ii. STRATIGRAPHICAL POSITION.—ANALOGIES FROM CENTRAL FRANCE
In the history of opinion regarding the relative position of the Tertiary eruptive rocks, no feature is so remarkable as the universal acceptance of the misconception regarding the place of the acid protrusions. In tracing this mistake to its source, we find that it probably arose from the fact that along their line of junction the granitoid masses generally underlie the basic. This order of superposition, which would usually suffice to fix the age of two groups of stratified rocks, is obviously not of itself enough to settle the relative epochs of two groups of intrusive rocks. Yet it has been assumed as adequate for this purpose, and hence what can be proved to be really the youngest has been placed as the oldest part of the Tertiary volcanic series.