CHAP. XLV 
THE ACID ROCKS 
365 
Professor Yon Lasaulx fully described tlie “ trachyte ” or rhyolite of 
Antrim . 1 
This interesting series of rocks embraces a greater variety of petro- 
graphical characters than any other portion of the British Tertiary volcanic 
rocks. On the one hand, it presents thoroughly vitreous masses, some of 
which in their colour, lustre and microscopic structure remind us of recent 
obsidians. On the other hand, it affords coarsely crystalline compounds, to 
which no other name than granite can be assigned, and which, did we not 
know their geological position, might almost be classed with some of the 
most ancient eruptive rocks. Between these two extremes abundant grada- 
tions may be found, including beautiful spherulitie rocks, felsites and 
rhyolites. 
In dealing with such a series of intrusive rocks, we again encounter the 
difficulty of reaching certainty as to their relative dates of eruption, since in 
each case all that can usually be affirmed is that the intrusive mass is 
younger than that into which it is injected. It is quite possible that pro- 
trusions of acid rocks occurred at intervals during the accumulation of the 
basic masses, as may perhaps be inferred from the rliyolite-tuffs and con- 
glomerates of Antrim and from the occurrence of fragments of siliceous lavas 
in the gravels near the base of the basalt-plateau of Mull, and in the 
agglomerates of that island as well as of other districts . 2 It is probable, 
therefore, that at the time when the basalts of the plateaux were emitted, 
there existed, within reach of volcanic explosions, masses of granophyric, felsitic 
or rhyolitic rocks, fragments from which were shot up the funnels of discharge. 
That portions of these rocks were actually intruded into the basalt-sheets 
before the building up of the plateaux was completed appears to be proved 
in Antrim. Elsewhere, however, no evidence has yet been obtained of any 
such intrusion until after the close of the plateau-period. On the contrary, 
in every case where the relative ages of the rocks can be fixed, the acid are 
younger than the basic protrusions. 
The only known exceptions to this rule are the latest basalt-dykes. 
Hence, while amid the large and varied series of acid rocks, which no doubt 
represents a wide interval of time, some may belong to comparatively early 
epochs in the protracted volcanic period, the actual available evidence places 
the emission of these rocks, as a whole, towards the end of the volcanic 
history. This evidence I shall bring, forward in full detail, since it 
necessitates an abandonment of what has been the general belief in regard 
to the relative ages of the rocks. 
1 Tschermak’s Min, und Petrog. Mittheilungcn, 1878, p. 412. The chemical composition of 
this rock and its place among the rhyolites had already been determined by E. T. Hardman 
from analysis, Journ. Geol, Soc. Ireland, vol. iii. (1871), p. 82. 
2 Reference may also again be made to the agglomerates of Strath, Skye, which contain in 
some parts abundant fragments of acid rocks that closely resemble some of the masses of grano- 
phyre which disrupt these agglomerates. 
