476 Proceedings of the Royal Irish Academy. 



The reasons for classifying tlie rocks of group 1 as Arenig are given 

 in a Paper already read before the Academy, "On some supposed 

 Cambrians in Co. Tyrone and E^.E. Mayo."^ 



Attention is now directed to the rocks of the Pomeroy series, 

 which are evidently much newer than the metamorphic rocks north 

 of them {Arenig group), although both groups are called Lower Silu- 

 rian, in the Geological Survey Memoir descriptive of the district. The 

 fossils of this series are of types indicating an age similar to the 

 English Caradoc-Bala series : this, although important, is not conclu- 

 sive evidence, on account of the zones of English Cambro-Silurian 

 fossils occurring in the Irish Silurians. All the evidence on both 

 sides was therefore given. 



The point of greatest interest with respect to the Pomeroy series 

 is to determine the relation between them and the overlying ones of 

 the "Lower Old Red Sandstone" type : we believe this to be one of 

 unconformability. Portlock, on the contrary, says in his Report that 

 the latter seem to lie conformably on the former. His boundary of 

 the Pomeroy strata has been left substantially unaltered by the Geo- 

 logical Survey. If the southward part of his boundary between the 

 two groups now in question in Shanmaghry were rightly placed, his 

 conclusion would, in all probability, be correct, because the rocks 

 above and below the geological horizon at that place evidently form a 

 continuous sequence. But the more detailed maps which accompanied 

 our Report (of which PI. XVI., fig. 1, is a reduction) show that Port- 

 lock's boundary near Aghafad is probably incorrect, as the faults that 

 shift the boundary in that part are ignored. He has there included in 

 the Pomeroy series rocks which really belong to the " Lower Old Red 

 Sandstone" type. We believe that the boundary should be north of 

 Shanmaghry, viz., at Aghafad (see PI. XYI., fig. 2). The section at 

 that place is the only nearly continuous one to be seen ; and although 

 it is not quite conclusive, it certainly suggests an unconformability 

 at the red conglomerate there exposed. Elsewhere Portlock's boun- 

 dary would imply an unconformability between the two groups of 

 strata in question. The long narrow strip of " Lower Old Red Sand- 

 stone" which he supposed to extend along the east side of the Pomeroy 

 strata between them and the Carboniferous sandstone, if it really 

 existed, would in all probability lie unconformably on the Pomeroy 

 strata. But we cannot avail ourselves of this confii'mation of our po- 

 sition, for all the evidence that we could collect goes to show that the 

 Carboniferous sandstone lies directly on the Pomeroy strata, without 

 any intervening " Lower Old Red Sandstone." Along the western 

 side of the exposure of the Pomeroy strata, where Portlock's boundary 

 between the two groups noAV in question is correctly drawn, no ma- 

 terial for a conclusion, one way or the other, respecting the relation 

 of those groups can be obtained, on account of the numerous faults in 

 that neighbourhood. 



^ proceedings, Eoyal Irish Academy, ser. ii, vol. iii.. Science. 



