GEOLOGICAL SOCIETY OE DUBLIN'. 
11 
range of the Silurian rocks, which extend continuously in patches from 
shore to shore, east and west across the island. These grits might he 
higher or lower in the series, a point I did not venture absolutely to de¬ 
termine, though I considered them to he of more recent origin than the 
schistose rocks which lie to the east of the country, hut still a portion, 
one and indivisible, of a lengthened and continuous sequence. Upon 
further examination however, finding them to he, as I believed, confor¬ 
mable with the undoubted Silurian strata, which extend south-west from 
Smerwick Harbour to the Blasket Islands, these strata being now con¬ 
sidered to he of Wenlock and Ludlow age, as I am informed by Mr. 
Jukes; and independently at another point finding them to conform to a 
small outlier of Silurian which occurs on the south coast of the peninsula 
at Coosatorig,—I felt that I had such a corroboration of the conclusion 
at which I had gradually been arriving, that I had no further hesitation 
in at once laying down the Glengarriff grits on my Map as belonging to 
the Silurian system, whether the same consisted of an upper or a lower 
series; and as the unconformity between the upper and lower Silurian 
(the latter of which Mr. Jukes proposes to call Cambro-Silurian), as ex¬ 
isting elsewhere, does not appear to me to affect the questidh relative to 
the position of the Glengarriff grits, which are associated with and con¬ 
form to the former,—I do not consider it necessary to advert to that 
discussion further than to observe, that should such unconformity be 
well established over wide areas, it may ultimately be necessary to in¬ 
troduce a new systematic term, which at present would not seem to be 
sufficiently warranted, at least in Ireland. 
Before passing to the consideration of the Glengarriff grits of the 
Glengarriff district, I wish to remark, that the fact of the derivative cha¬ 
racter of these rocks, as noticed by Mr. DuUoyer, who finds them to be 
composed of the debris of the underlying Silurians, containing fossili- 
ferous pebbles in some localities), is not conclusive, in my mind, of their 
being an independent formation, whether to be called Devonian or some¬ 
thing systematically new,—as I think that cases of derivative rocks some¬ 
times occur in the same continuous series, and that such cases are rather 
to be expected, if we suppose the existence at points not far distant, of the 
contemporaneous operation of agencies of denudation and deposition. 
I should have mentioned that the fossiliferous Silurian strata of the 
Dingle district consist of alternations of brown, gray, anc^green beds, 
containing upper Silurian fossils, purple slates, reddish and yellowish 
shales and sandstones, with brown sandstones, and occasional brecciated 
conglomerates, which latter are also found associated with the upper Glen¬ 
garriff group of strata, which I now propose to consider. If we suppose 
that we have overcome the chief, or all the difficulties, that we have had to 
encounter in endeavouring to interpret the geology of the Dingle district, 
we will find that upon entering the territories of the Glengarriff grits 
of the Glengarriff district, we have, as it were, accomplished nothing to¬ 
wards a harmonious view of the rocks composing the south-west of 
Ireland. 
Here at the very threshold we are apparently met with an insur- 
