242 
THE SILURIAN VOLCANOES 
BOOK IV 
from Belfast Lough to the southern coast-line of Waterford, even as far as 
l)ungar\an Harbour. W^ith the same lithological types of sedimentary 
deposits as in othej- parts of the United Kingdom, they carry with them 
here also their characteristic records of contemporaneous volcanic action. 
Though nowhere piled into such magnificent mountain-masses as in West- 
moreland and North Wales, these records become increasingly abundant and 
interesting as they are traced southwards, until they are abruptly terminated 
by the coast-line along the south of the counties of Wexford and Waterford. 
While much remains to be done, both in the field and in the laboratory 
and microscope-room, before our acquaintance with the Irish Silurian volcanic 
locks is as complete as our knowledge of their equivalents in other portions of 
the United Kingdom, a serious preliminary difficulty must be recognized in the 
fact that the several geological horizons of these rocks have only been ap- 
proximately fixed. Great difficulty was experienced by the Geological Survey 
in diawing any satisfactory line between the Llandeilo and Bala formations. 
This arose not so much from deficiency of fossil evidence as from the way 
in which the fossils of each group seemed to occur in alternating bands 
in what were regarded as a continuous series of strata. Indeed, in some 
localities it almost appeared as if the occurrence of one or other facies of 
fossils depended mainly on lithological characters indicative of original 
conditions of deposit, for the Llandeilo foi'ms recurred where black shales 
set in, while Bala forms made their reappearance where calcareous and 
gritty strata predominated.' More recent work among the Silurian forma- 
tions in England and Scotland, however, indicates that the parallel repetition 
of the two types of fossils is due to rapid and constant plication of the 
rocks, whereby the two formations, neither of them, perhaps, of great thick- 
ness, have been folded with each other in such a way that without the 
evidence of an established sequence of fossils, or the aid of continuous 
sections, it becomes extremely difficult to make out the stratigraphical order 
in any district. When the ground is attacked anew in detail, with the 
assistance of such pala;ontological and lithological horizons as have per- 
mitted the complicated structure of the southern uplands of Scotland to be 
unravelled, we may be enabled to tabulate the successive phases of the 
volcanic history of the region in a way which is for the present impossible. 
We have as yet no pahcontological evidence that in the Silurian region of 
the east of Ireland, which extends from Belfast Lough to the south coast of 
County Waterford, any of the anticlinal folds bring up to the surface a 
portion of the Lower Arenig formation, though j)ossil)ly some of the lowest 
vLsible strata may be of Upper Arenig age. A con.siderable part of the 
region must be referred to the Llandovery and other Upper Silurian 
formations, but the precise limits of the two divisions of the Silurian 
system have not yet been determined, except for the region north of Dublin, 
which has recently been re-examined for the Geological Survey by Mr. 
F. W. Egan and Mr. A. M'Henry. 
J Jukes was disposed to regard the two faunas as essentially coeval, but inhabiting different 
kinds of sea-bottom. See his note, Kxplanation of Sheets 167, 168, 178, 179, p. 30. 
