GEOLOGICAL SOCIETY OP DUBLIN. 
159 
will be found in few places, within a small compass, such a variety of 
interesting' examples of study as we have presented on this shore, the 
convoluted strata of which form, in fact, a natural model, as it were, of 
abstract Geology. 
Immediately above the upper carboniferous limestone, and confor¬ 
mable with it, lie beds of the millstone grit series, or, as in reference 
to Ireland they might be called, the marine coal formation, from the 
occurrence of marine fossils in abundance, which are characteristic of 
the carboniferous limestone series generally. These fossils consist of 
Zoophytes, Crustacea, Brachiopoda, Conchifera, Gasteropoda, and Cepha¬ 
lopoda. This series in Ireland is divisible into an upper and lower 
sandstone or grit (in which are frequently found casts of Lepidodendra 
and Stigmariee), having a considerable thickness of fine-grained, dark 
gray shale interposed, in which latter numerous beds of argillaceous 
ironstone are intercalated; and it is these shales which contain such a 
numerous variety of marine fossils of the ordinary carboniferous type, 
as I have mentioned above, the Aviculo-pecten papyraceus characte¬ 
ristic of the formation, as in England, being abundant throughout. As 
in the case of those of the Calp series, so in the finer-grained shales of 
this formation, casts of Posidonia in great profusion are found to occur, 
and the several varieties of form in both cases are identical, such as Becheri, 
lateralis , and tuberculata, though some of those of the smaller sort may 
possibly be new, and others would appear to be of the genus Inoceramus. 
Eerns, Sigillarice and Lepidodendra, are also abundant, and we occasionally 
meet with interstratifications of flaggy fossiliferous limestone. The locali¬ 
ties in which Posidoniae most abound are near Ennistymon, in the county 
of Clare; Ballybunnion, in the county of Kerry; Braulieve Mountains, in 
the county of Sligo; Cuilcagh Mountain and the Alteen Eiver, in the 
county of Cavan; Corry,* near Drumkeeran, in the county of Leitrim; 
and Mullaun and Carrownanalt, near Keadue, with the Munterkenny 
Mountains, in the county of Boscommon. 
Before closing this communication, I should wish to mention that in 
making researches relative to the persistence of the genus Posidonia, I 
have ascertained its occurrence at the base of the carboniferous series in 
the suite of rocks to which I have given the name “carboniferous slate,” 
in the Geological Map of Ireland; and I think it may be desirable to 
make some reference to this fact, as well as to give a short description 
of the strata of this lower series. These rocks (typical in the south of 
Ireland) consist of cleavable slates or shales, the colour of which varies 
from dark gray to greenish or yellowish gray,—the former, however, 
being the predominating tint. The shale beds are usually interstratified 
with compact sandstone, and occasionally with limestone, the latter being 
sometimes of considerable thickness; the whole lying conformably be¬ 
neath the lower limestone, and similarly resting on the sandstone, for 
which I have hitherto adopted the term “yellow sandstone,” to distin¬ 
guish it from the subjacent and conformable beds of the true, and pro- 
* See Plate XVIII., Fig. 3. 
