CHAF. XLVII 
THE ACID ROCKS OF S LI EVE GULLION 
425 
Throughout its whole extent, it is found to enclose these stones, which here 
and there become so numerous as to form the main bulk of the mass, leaving 
only a limited amount of quartz-porphyry (rhyolite) matrix to bind the 
whole into an exceedingly compact variety of breccia. Occasionally the acid 
rock cuts through the ordinary clastic agglomerate, as. may be well seen 
on the southern face of Tievecrom. 
A specimen of this porphyry with its enclosed fragments, which was 
collected by me from above the old tower at Gleudovey, Carrickbroad, 
has been sliced and examined by Mr. Whitts under the microscope, and is 
thus described by him : “ The large fragment in this slide consists of 
ophitic olivine-dolerite full of large phenocrysts of olivine. It is broken up 
and penetrated by veins of quartz-porphyry, rich in quartz, which exhibits 
a beautiful flow- structure. The felspars and augite of the dolerite do not 
appear to have suffered much alteration at the margin of the fragment, but 
the olivines are much serpen tinized, the serpentine passing into a border of 
actinolite which runs in veins into the neighbouring rock and even passes 
out into the quartz-porphyry at the junction, impregnating it with actino- 
lite and chlorite for some distance. A few particles of basalt also occur 
and a portion of a granite-fragment comes into the slide, from the edge of 
which a piece of biotite lias floated off into the quartz-porphyry.” 
The essentially non-volcanic material of the agglomerate shows, as Mr. 
Nolan pointed out, that it was produced by aeriform explosions, which blew 
out the Silurian strata and granite in fragments and dust. These discharges 
probably took place either from a series of vents placed along a line of 
fissure running in a north-westerly line, or directly from the open fissure 
itself. Possibly both of these channels of escape were in use ; detached 
vents appearing at the east end and a more continuous discharge from the 
fissure further west. 
After the earliest explosions had thrown out a large amount of granitic 
and Silurian detritus, andesitic lava rose in the fissure, and solidifying there 
enclosed a great deal of the loose fragmentary material that fell back into 
the chasm. Subsequently, and on 'a more extensive scale, a much more acid 
magma ascended from below, likewise involving and carrying up a vast 
quantity of loose stones, among which are pieces of basalt and dolerite. 
No evidence remains as to the extent of the material discharged over 
the surface from this fissure. Denudation has removed all the surrounding 
fragmental sheets as well as any lava that may have flowed out upon or 
become intercalated among them. There remains now only the cores of the 
little necks at the east end, and the indurated agglomerate and lava that 
consolidated along the mouth of the fissure or vents. 
This is the only example of such a line of fissure-eruption which has 
yet been met with in the British Isles. Its connection with the eruptive 
masses of Slieve Gullion and Carlingford links it with the Tertiary volcanic 
series. But no evidence appears to remain regarding the epoch in the long 
volcanic period when the eruptions from it took place. They may possibly 
date back to the time of the plateau-basalts ; but the abundant acid magma. 
