CHAP. XIX 
THE ULSTER ERUPTIONS 
315 
of andesitic lavas. The Cusheudall rocks become coarser as they are traced 
northwards into lower members of the series, while at the same time the 
proportion of porphyry-debris in their constitution diminishes, and materials 
from the metaniorphic series take its place. Thus at Cushendun the per- 
centage of quartz-pebbles rises to 70 or 80. These blocks, ot all sizes up 
to two feet or more in diameter, are admirably rounded and smoothed, like 
those in the Stonehaven section and those among the conglomerates at the 
south end of Cantyre. Fragments of the porphyry, however, still continue to 
appear, and the matrix show's an admixture of the finer detritus of that rock. 
I may remark in passing that no conglomerates of the Old lied Sandstone 
show more strikingly than these at Cushendun the effects of mechanical crush- 
ing subsequent to deposition and consolidation. In many parts of the rock it 
is hardly possible to find a rounded block that has not been fractured. Some 
of them, indeeil, may be seen cut into lialf a dozen slices, which have been 
pushed over each other under the strain of strong lateral or vertical pressure. 
In the interior of the country, after passing over the broad Tertiarj' 
basaltic plateau of Antrim, W'e come upon a large area of Lower Old lied 
Sandstone in Tyrone. It stretches from Pomeroy to Loch Erne, a distance 
of about 30 miles, and is about 12 miles broad. In lithological character 
the strata of this tract exactly resemble parts of the deposits of Lake 
Caledonia in Central Scotland. They include also a volcanic series which, 
dow'ii to the smallest points of detail, may be paralleled in the sister island.^ 
This interesting westward prolongation of the volcanic record consists of 
a number of outlying patches confined to the eastern part of the district. 
The largest of these patches lies to the south of Pomeroy, where it forms 
a line of hills about four miles long, and covers an area of some five square 
miles. The rocks consist of successive sheets of andesite-lavas. These, as 
a rule, are not markedly cellular, though they include some characteristic 
amygdaloids. A distingushing feature of some of the sheets is their remark- 
ably well-developed liow-structure. Thus on Sentry Box, at the north-western 
end of the ridge, the fissility resulting from this structure so perfectly divides 
the rock into parallel flags that the material might easily be mistaken for a 
bedded rock. Where this structure Iras been produced in a cellular lava, 
the cavities have been drawn out and flattened in the direction ot flow. 
I have not observed true tuffs in any of the sections traversed by me 
in this district. But the conglomerates furnish abundant evidence of the 
contemporaneous outpouring of the lavas. Thus, in a brook a little west of 
lleclain, five miles south of Pomeroy, the section shown in Fig. 84 may be 
seen. At the base lies a coarse conglomerate (a) largely composed of 
andesite-debris, the stones being here, as elsewhere in the district, well 
rounded. Then comes a series of green and reddish highly-felspathic sand- 
stones (&), followed by an exceedingly coarse conglomerate (c), formed mainly 
of the debris of andesites, especially lumps of slag. Some of the stones 
1 This area of Old Red Sandstone is represented on Sheets 33, 34, 45 and 46 of the Geological 
Survey of Ireland, and the igneon.s I'ochs are described in the Memoirs on Sheets 33 (1886, p. 17) 
by Mr. J. R. Kilroe, and 34 (1878, p. 16) by Mr. J. Nolan. 
