CHAP. XIV 
LLANDEILO AND BALA ERUPTIONS OF IRELAND 
243 
Tliese observers have ascertained that, as in Southern Scotland, by far 
the larger part of the Silurian region of the north-east of Ireland is occupied 
by strata belonging to the upper division of the system. The Lower Silurian 
formations, including the Llandeilo and Bala groups, form a belt varying up 
to six miles in breadth, which stretches from the coast of Down, between the 
mouth of Belfast Lough and Copeland Island, in a south-westerly direction 
to near the valley of the Shannon in County Longford. South of this belt 
the Lower Silurian rocks rise to the surface only here and there on the 
crests of anticlinal folds, and it is in these scattered “ inliers ” that the vol- 
canic and intrusive rocks are found. So far as the available evidence goes, 
the volcanic history of this part of Ireland is entirely to be assigned to , 
Lower Silurian time, and more especially to the interval between the begin- 
ning of the Llandeilo and the close of the Bala period. I must for the 
present content myself with this general limit of geological chronology, and 
make no attempt to trace the relative antiquity of the igneous rocks in the 
several districts in which they are distributed.’^ 
Viewing the volcanic region of Eastern Ireland as a whole, we are first 
struck by the feebleness of the manifestations of eruptivity in the north, and 
their increasing development as we advance southwards. At the northern 
end of the Silurian area in County Down, thin bands of “ felstone ” and 
“ ash ” have been mapped by the Geological Survey as interstratified with 
the highly inclined and plicated Silurian rocks.^ As the latter are plainly 
a continuation of the strata which have been mapped out zone by zone in 
the south of Scotland, their igneous intercalations may be boked upon as 
probably equivalents of some of those in the Silurian districts of Wigton- 
shire and Kirkcudbrightshire. But in County Down no representative has 
yet been detected of the Arenig and Llandeilo volcanic series of the southern 
uplands of Scotland. Nor has more precise petrographical examination con- 
firmed the reference of any of the igneous rocks in the Silurian area of that 
district to truly contemporaneously intercalated volcanic rocks. All the 
eruptive material appears to be of an intrusive character. It occurs in the 
form of dykes of lamprophyre or mica-trap belonging to the groups of minettes 
and kersantites. Nothing definite is known of the age of these intrusions ; 
they are possibly referable to the time of the Lower Old Bed Sandstone. 
Ear in the interior several bands of “ felspathic ash and massive 
agglomerate ” are shown on the Survey map as running through the counties 
of Monaghan and Cavan.* In one locality south of the Drumcalpin Loughs 
a large exposure of this ash is visible : “ brown crumbly beds, with small 
' The task of revising the Irish maps and tracing out the respective areas of Upper and Lower 
Silurian rocks over the whole island is now in progress by the Geological Survey, Mr. Egan and Mr. 
‘Henry being entirely engaged on it. 
■- See Sheet 49 Geol. Survey, Ireland, and E.xplanation thereto (1871), pp. 16, -37, 39. The so- 
called “ a, dies ” of the Explanation are probably parts of dykes which have been more or less 
crushed. 
“ Ouklc to the Collection of Rocks and Fossils belonging to the Ceological Survey of Ireland, 
by Messrs. 31‘IIenry and Watts, Dublin, 1895, |i. 74. 
* Sheet 69 Geol. Survey, Ireland, and Explanation of Sheets 68 and 69, pp. 9, 13, 15. 
