422 
THE TERTIARY VOLCANOES 
BOOK VIII 
two groups of rocks was iirst clearly recognized by Griffith, who showed that 
the granite (granophyre) is intruded into the gahbro. 1 Professor Haughton 
subsequently confirmed this observation, and proved the post-Carboniferous 
date of the intrusive materials, which he compared with those of Skye. 2 The 
general distribution of the rocks was traced out in some detail by the 
Geological Survey, and described in the official Memoirs , 3 More recently 
the district has been examined by Professor Sollas, who, bringing the 
photographic camera and the microscope to the aid of field-geology, has 
elucidated the structure and relations of the rocks, and has obtained abun- 
dant evidence that the acid and basic rocks maintain there the same relative 
order as among the Inner Hebrides. 4 
One of the first features in this tract of country to arrest the eye of the 
geologist is the situation of this centre of protrusion and that of Slieve 
Gullion along a north-west line, coincident with the general direction of the 
numerous basic dykes of the region. Whether or not the successive intru- 
sions took place contemporaneously in the two areas, they have followed each 
other in the same order. In the Barnavave district the igneous rocks occupy 
an area of about 20 square miles. They consist of a central and chief mass 
composed of acid materials, which have risen through the basic rocks now 
found as an interrupted ring round them. 
In his more recent examination, Prof. Sollas has devoted special attention 
to the influence of the solvent action of the acid magma upon the basic rocks 
and upon its own composition and structure. Besides confirming the work of 
previous observers as to the order of appearance of the two kinds of material, 
he has obtained evidence that the gahbro had not only completely solidified, 
but was traversed by contraction-joints, possibly even fractured by earth-move- 
ments, before the injection of the granophyric material. He found that this 
material, like that of the Inner Hebrides and St. Kilda, must have been in 
a state of great fluidity at the time of its intrusion, and made its way into 
the minutest cracks and crevices. In observing the solvent action of the 
granophyre, he ascertained that this action took place even in comparatively 
narrow dykes, which probably consolidated at no great depth beneath the 
surface. 5 
c. The Slieve Gullion District . — This area is separated from that just 
described by a narrow strip of Silurian strata, so that its isolation as a 
separate igneous district is complete. It will be observed from the map to 
continue the same north-westerly line as the Slieve Foye tract, the two 
together running in that direction for a distance of some 16 miles. It 
is interesting to note the adoption of this predominant north-westerly trend 
even by eruptive masses which were mainly of acid material. 
1 Journ. Gcol. Soc. Ireland (1843), p. 113. 
2 Quart. Journ. Gcol. Soc. vol. xii. (1856), p. 171 ; xiv. p. 300 ; and Journ. Gcol. Soc. Ireland 
(1876), p. 91. 
3 Sheet 71 of the Geol. Surv. Ireland, and accompanying Explanation. These were the 
work of Mr. W. A. Traill. 
4 Trans. Roy. Irish Acad. vol. xxx. (1894), p. 477. This is part i. of wliat is intended to be a 
series of papers. 5 Op. cit. 
