CHAP. XI.V 
THE ACID ROCKS— ANALOGIES FROM FRANCE 
37i 
Rhyolite, (Quartz- Trachyte ). — This rock lias been abundantly erupted 
in north-east Ireland, where it rises in occasional bosses among the plateau- 
basalts. 1 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 Yon Lasaulx as those of a typical quartz-trachyte rich in 
tridymite, and containing large crystals of glassy sanidine, isolated narrow 
laths ot plagioclase (probably andesine), grains of smoky-grey quartz, partly 
bounded by dihexahedral faces, and a few scattered flakes of a dark-coloured 
mica. 1 he groundmass is micrograuitic, and under a high power is resolv- 
able into a confused aggregate of minute microlites of felspar, with inter- 
stitial quartz-granules." More recently a detailed investigation of the petro- 
graphy 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 spherulitie structures. 3 
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 
fheir line of junction the granitoid masses generally underlie the basic, 
f his 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 mlequate for this purpose, and hence what can lie proved to be really the 
Joungest has been placed as the oldest part of the Tertiary volcanic series. 
Maeculloch, who showed that his “ syenites ” and “ porphyries ” had 
uivaded the Secondary strata of the Inner Hebrides, and must therefore be 
°t younger date than these, left their relations to the other igneous rocks of 
the region in a curiously indefinite position. He was disposed to regard 
them all as merely parts of one great series ; and seems to have thought that 
1 Fragments of acid rook were detected by Prof. Cole in the gravel among the Ardtun basalt 
0 Mu 1] 5 as already noticed on p. 212. 
* Tschermalc’s Mm. und Pet. MUtheil. 1878, p. 412. 
Sdentif. Trans. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77. This paper gives an excellent 
a ccount of the microscopical character and mineralogical and chemical compositions of these 
rocks. 
