Cahercoalisfv Knockgrecuv DerkHill 
48 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
chiefly through the limestones that emerge from 
below these volcanic sheets. One of the most con- 
spicuous of them, Derk Hill (Fig. 195), rises to a 
height of 781 feet above the sea, and comes through 
the bedded andesites, as represented in Fig. 196, 
which gives, in diagrammatic form, the general 
structure of the Limerick volcanic basin. Around 
the northern side of the basin a smaller number of 
necks has been observed, consisting of similar acid 
rocks. 
A few of the necks appear to be filled with 
volcanic agglomerate. Here and there detached 
patches of fragmental volcanic material have been 
shown on the Survey maps, and referred to in the 
Explanations, as if they were outliers of the bedded 
tuffs ; though in some cases the coarseness of their 
materials and the want of any distinct bedding, 
together with the absence of any indication of their 
relation to the nearest limestones, have evidently 
offered considerable difficulty in their mapping. 
One of the best examples occurs about two miles 
to the south-east of the village of Oola. The 
boundaries of this patch, as put on the map, are 
confessed to be “ entirely speculative.” It was only 
seen on the side of the railway where it- appeared 
as “ a very coarse brecciated purple ash.” 1 
On comparing the maps of the Limerick basin 
with those of the Carboniferous districts of' Scotland, 
the main difference will probably be acknowledged 
to be the absence of any recognizable sills in the 
Irish ground. That no sills actually occur, I am 
not prepared to affirm. Indeed some of the more 
acid rocks, both outside the basin and among the 
rocks of the older volcanic group, appeared to me 
during my traverses of the ground to have much of 
the character of sills. A more critical examination 
of the area would not improbably detect some truly 
intrusive sheets which have hitherto been mapped 
among the interstrati fied lavas. Some appear to 
exist among the surrounding Lower Limestones. 
An intrusive mass, like a sill or dyke, is repre- 
sented on the Geological Survey Map as traversing 
the Coal-measures in the inner basin south of 
Ballybrood. But as the strata are on end along 
its southern margin, it may possiby be only a 
1 Explanation of Sheet 154, p. 25. 
