CHAP. XXX 
VOLCANIC BRECCIAS OF COUNTY CORK 
49 
portion of the upper volcanic series which has been thrown into its present 
position by one or more faults. 1 
8. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK 
In the south-western headlands of Ireland, from Bear Island to Duisey 
Island, various igneous rocks have been traced on the maps of the Geological 
Survey. They have been described as consisting of “ greenstone,” “ felstone 
and “ ash ” or “ breccia,” and as including both interstratified and intrusive 
masses. 2 If contemporaneous with the strata in which they occur, they 
would prove the existence of a group of volcanic rocks in the Carboniferous 
slate, or lowest division of the Carboniferous system. After an examination 
of the coastline I came to the conclusion that while there is undoubtedly 
evidence of former volcanic activity in this part of Ireland, no proof has 
been obtained that the eruptions occurred in the Carboniferous period. The 
felsites and dolerites appeared to me to be all intrusive, the former having 
certainly been injected before the terrestrial movements that have disturbed 
the rocks, for some of them share very markedly in the cleavage of the 
region. The dolerites and diabases, on the other hand, so far as I observed, 
are not cleaved, and are thus probably of later date. . , „ 
The most interesting rocks are undoubtedly the “ ash ” and “ breccia, 
for they are obviously of volcanic as distinguished from plutomc origin. 
On the coast north of White Bull Head, a bed of volcanic breccia 
may be seen made up of rounded and angular fragments of different sand- 
stones, shales and limestones, with pieces of felsite and andesite wrapped up 
in a dull-grey fine-grained sandy felspathie matrix. The rock weathers 
with a rough or rugged surface, owing to the dropping out of the more 
decomposable stones.' This bed, about five feet thick, runs with the bedding 
of the strata around it, and like these dips S.S.W. at an angle of 70°. If no 
other evidence were obtainable, this breccia would be naturally set down as 
a truly interstratified deposit of volcanic detritus. A short distance fiom 
it, a second, rather thicker band of similar material occurs, specially 
distinguished by its abundant worn crystals of hornblende, sometimes three 
inches in diameter, as well as large crystals of- muscovite. These minerals 
are not unknown elsewhere in volcanic agglomerates. The occunence 
of lumps of augite in the vents of Upper Old Bed Sandstone age m 
Caithness has been already alluded to, and a still larger series of ejected 
minerals will be shown in a later chapter to characterize the younger necks 
of Central Scotland. 
In parts of its course, this second band appears to run so perfectly 
parallel with the bedding of the strata between . which it lies that the 
■observer would readily believe it to be a part oi the same series of 
deposits, and might therefore regard it as affording good evidence of 
J Sheet 154 and Explanation to the same, p. 24. , 
2 See Sheets 197 and 198 of the Geological Survey of Ireland, and the Explanation 
Sheets by Messrs. Jukes, Kinalian, Wilson, and O’Kelly, 1860. 
* ill 
VOL. II 
