On the Laurentian Rocks of Donegal ond of other parts of Ireland. 255 
margin of the Silurian and Devonian series, and for which I have suggested the 
term “ Devono-Silurian.” But in the neighbourhood of Pomeroy there occurs a 
series of greenish slates and flags in which Portlock discovered a remarkable series 
of Lower Silurian fossils, which have made them well known amongst geologists as 
“the Pomeroy Beds.” The fossils from this series, having received accession from 
the examination of the officers of the Survey, have recently been revised and 
tabulated by Mr. Baily, F.G.S., Acting Palzeontologist to the Irish Survey, and are 
given at length in the Explanatory Memoir to accompany Sheet 34 of the Geological 
Map. The list is exceedingly copious, and, in Mr. Baily’s opinion, indicates that 
the beds are referable to the “ Caradoc” or “ Bala” stage of the Silurian series.* 
The relations of these beds to the metamorphic rocks lying to the north and west 
of their position is unfortunately very obscure ; and on the Survey map the two 
formations are separated by faults, by which the “ Bala beds” are brought down 
against the metamorphic rocks. Weare, therefore, driven to other kinds of evidence 
than that of superposition of strata, in order to determine, as far as possible, the 
geological age of the metamorphic series. The question is whether these rocks 
belong to the Laurentian (“‘ Pre-Cambrian ”) or to the metamorphosed Lower Silurian 
series. In Griffith’s map they are represented under the same colouring and 
lettering (Ye.) as the rocks of the Ox mountains in county Sligo, which I have already 
referred to the Laurentian series. Sir R. Griffith was, evidently, of opinion that they 
are of older age than, and that they underlie, the great metamorphic series, which 
occupies the greater portion of the county of Londonderry, from Lough Foyle and 
Lough Swilly southwards ;—beds forming the upper portion of the Lower Silurian 
metamorphic series. In this case the lower beds which rest on the Laurentian 
series of Donegal might possibly be represented by the Tyrone rocks. There is, 
however, so little resemblance in the characters of the two sets of rocks that this view 
appears highly improbable. So far the evidence is in favour of their Laurentian age. 
On the other hand, as the fossiliferous beds of Pomeroy belong to the Bala series, 
it may be supposed that they overlie several thousand feet of representatives of the 
“Llandeilo” and, possibly, “ Arenig” beds ; and these, being deeper seated, might 
have been metamorphosed, while the overlying shallower beds might have remained 
(as at Pomeroy) unaltered ; these lower beds would be concealed along the lines of 
fault above described, and only appear on the upthrow side where they are exposed 
by denudation. 
I have thus endeavoured to put fairly the uncertainties attending the determina- 
tion of the geological age of the Tyrone rocks; and, on considering the whole 
question, I am inclined to concur in Dr. Hicks’ view of their Pre-Cambrian age, 
partly on stratigraphical, but chiefly on petrological, grounds. They may probably 
be viewed as an “‘ Upper Laurentian” series not represented in Donegal. Additional 
* Dr. Hicks (quoting from Mr. Kinahan) calls these beds ‘‘Cambro-Silurian,” a term which seems to 
have somewhat misled Dr. Hicks in his speculations regarding the age of the crystalline rocks. A 
reference to Survey Documents or to Portlock’s work would have shown him the exact age of the beds. 
TRANS. ROY. DUB. SOC., N.S., VOL. I. 2U 
