254 
THE SILURIAN VOLCANOES 
BOOK IV 
indicate that they were formed close to the shores of a land composed of 
schistose rocks, quartzite and granite, of which the mountainous tracts of 
Connemara are the last relics. 
A base to the volcanic series is found in the occasional uprise of a short 
axis of Llandeilo, or perhaps even upper Arcnig strata, containing hands of 
dark chert and black gi'aptolitic shales. Unfortunately the relations of these 
underlying rocks to the volcanic masses are not very clear, Ijeing obscured 
by superficial accumulations and also by faulting. It is thus hardly 
possible to be certain wliether they pass up conformalfiy into the base of the 
volcanic series, or are covered by it unconformably. 
The position of this isolated volcanic.district in the far west of Ireland, 
the abundance, variety and thickness of the erupted materials, and the definite 
intercalation of these materials in the Bala or highest division of the Lower 
Silurian series, acquire a special interest from the history of the nearest 
Silurian volcanic area which has now to be described — that of the western 
shores of the Dingle promontory. 
II. The Upper Silurian Series 
The latest volcanic eruptions of Silurian time yet definitely known took 
place during the accumulation of the Wenlock and Ludlow rocks in the far 
west of Ireland. Xo satisfactory record of any contemporaneous phenomena 
of a like kind has yet been met with in any other Upper Silurian district 
in the British Isles, unless at Tortworth in Gloucestershire, as above 
described. So far as at present known, only one centre of activitv has 
been preserved. It lies among the headlands of Kerry, where the land 
projects furthest west into the stormy Atlantic. The occurrence of volcanic 
rocks in this remote area and their- geological horizon have been clearly 
indicated on the maps of the Geological Survey. More than thirty years, 
however, have elapsed since some of the mapping was done, and we must 
therefore be prepared to find it, more especially in its petrography, capable 
of modification and improvement now. 
In the country known as the Dingle promontory, these traces of con- 
temporaneous volcanic rocks are to be observed at various localities and 
on several horizons. To tlie east, near Anascaul, on the northern shore 
ob Dingle Bay, some tufis occur in what are believed to be Llandoverv 
strata. But it is on the western coast, among the headlands and coves 
that lie to the north and south of Clogher Head, that tlie best sections 
are to be seen. The succession of the rocks in this locality was well 
worked out by Du Xoyer, and the Memoir prepared liy him, with the 
general mtroduction by Jukes, is an invaluable guide to the geologist who 
would explore this somewhat inaccessible region.^ The most important 
correction that will require to be made in the work arises from a mistake as 
to the true nature of certain rocks which were described as pisolitic tuffs, 
but which are nodular felsites. 
* Sheets 160 and 171 of the one-inch map, and Mernoii- on Sheets 160, 161, 171 and 172. 
