426 
THE TERTIARY VOLCANOES 
KOOK VIII 
which constitutes one of their distinguishing characteristics, suggests that 
they more probably belong to the later time when the main protrusions of 
acid material took place. They suggest that coeval with the uprise of the 
great domes of Slieve Gullion, Carlingford and the Mourne Mountains 
there may have been many superficial eruptions of which, after prolonged 
denudation, all trace has now been effaced. 
2. The Antrim Eegion 
Reference was made in Chapter xxxvii. to the occurrence of rhyolitic 
conglomerate and tuff between the lower and upper series of basalts in 
the Antrim plateau, and to the evidence furnished by these detrital 
deposits either that masses of rhyolite appeared at the surface, or that 
rhyolitic ashes were discharged from volcanic vents in the long interval that 
elapsed between the two groups of basalt. The further consideration of this 
question, and an account of the rhyolite bosses, were reserved for the 
present chapter, that they might be taken in connection with the other acid 
eruptions of Tertiary time in Britain. 1 
With one exception, all the known- protrusions of acid material in the 
Antrim area lie within the limits of the basalt-plateau (see Map. No. YTI.) 
They occur along a line at intervals for a distance of about 17 miles, from 
Templepatrick to a point four miles north of Ballymena. It is worthy of 
remark that here again the line of protrusion has a north-west trend. It 
not improbably indicates the position of a fissure up which the acid material 
rose at various points. 
The petrography of the rocks has been frequently discussed. They 
include several varieties of rhyolite, generally rather coarsely crystalline, but 
sometimes becoming compact, and even passing into dark obsidian. No 
, undoubted tuff occurs associated with them in any of the exposures, nor do 
the rhyolites anywhere display structures that point to their having flowed 
out at the surface." That the masses now visible may have communi- 
1 For an early account of the Antrim trachytic rocks, see Berger, Trans. Geol. Soc. iii. (1816), 
p. 190. Professor Hull lias described the Tardree rock in the Explanation to Sheets 21, 28 and 
29, Geol. Survey of Ireland (1876), p. 17, and has supposed it to he older than the basalts, referring 
it to the Eocene period (Physical Geology and Geography of Ireland, 2nd edit. (1891), pp. 87, 95). 
Duffin (quoted by Mr. Kinahan) believed that “the trachytes occur at the centre of eruption, 
and were probably poured out at the end of the outburst.” Du Noyer also (quoted by the same 
writer) thought them to he newer than the plateau-basalts, and to have lifted up masses of these 
rocks. Mr. Kinahan himself ( Geology of Ireland, p. 172) has pointed to the absence of any 
rhyolitic fragments between the basalts as an argument against the supposed antiquity of the 
acid protrusions. A petrographies! account of the Tardree rock is given by Von Lasaulx in the 
paper already cited, Tschermak’s Min. Pet. MUthr.il. (1878), p. 112. A more elaborate discussion 
of the petrography by Prof. Cole will he found in the Memoir above referred to (Sdentif. Trans. 
Pny. Dublin Soc. vol. vi. 1896), and the geological relations of the rocks are discussed by him in 
another shorter paper, Geol. Mag. (1895), p. 803. See also Mr. M‘Henry on the trachytic rocks 
of Antrim, Geol. Mag. (1895), p. 260, and True. Geol. Assoc, vol. xiv. (1895), p. 110. 
2 At Sandy Braes an exposure is visible of what at first might he thought to be a volcanic 
conglomerate, but closer examination shows the roek to consist of obsidian, which decomposes 
into a clay, leaving round sharply -defined glassy cores enclosed in the decayed material. The 
“handed rhyolites” do not exhibit any kind of flow-structure that may not he met with in 
