GEOLOGICAL SOCIETY OF DUBLIN. 109 



from Irish data. He says, in the work already quoted, Introduction, 



p. 28 : " Though the Devonian series of the Herefordshire type seems 



to pass downwards into the upper Silurian groups, it does not appear to 

 pass upwards into the Carboniferous. There is generally a paheontolo- 

 gical and physical gap between them, which is in many places obscurely 

 indicated by the upper conglomerates of the Old Red Sandstone." 



His upper conglomerates of the Old Red Sandstone of Herefordshire 

 appear to me to be the same as my lower conglomerates of the Carboni- 

 ferous. His physical and palseontological gap I take to be the uncon- 

 formability at this point, and the difference of fossil genera and species 

 above and below, of which I have already spoken. Two coincidences of 

 opinion more close than these could scarcely be met with on a subject of 

 this kind. 



I have been told that cases occur on the north border of the coal-field 

 of South Wales where the coal rocks, limestone, conglomerate, and all, 

 could be shown lying conformably on the Old Red Sandstone of Breck- 

 nock and Monmouth. This, in a few local cases, I can easily believe ; 

 for, in the rolls that occur in the older strata in so large an extent, it is 

 very probable that the base of the Carboniferous formation, which has a 

 remarkable persistence here on the great scale, may coincide now and 

 then with the underlying undulations at an anticlinal or synclinal point, 

 though three-fourths of the observed cases along the junction be posi- 

 tively unconformable. 



Indeed, the general unconformability of those two may be inferred 

 from a mere inspection of the Map of Siluria itself, with the dips marked 

 on it from the new one-inch Map of England. In Herefordshire and in 

 Brecknock the general strike of the so-called Old Red Sandstone is S.W., 

 and the dip S.E. The strike of the Carboniferous rocks of South Wales 

 may be taken in a general way, in the vicinity of the Yans of Brecon, 

 by the outcrop of the limestone along its northern escarpment between 

 Abergavenny and Llandeilo, and this is east and west. Here the two 

 strikes clash ; they do not coincide, but make an angle with one another 

 of about forty or fifty degrees. This shows that the upper or Carboni- 

 ferous beds must lie unconformably on the lower brown beds, otherwise 

 the strikes of the two groups should be parallel. 



The Old Red Sandstone of Scotland, being a part of the subject, calls 

 for an observation. I cannot speak of this except from analogy. It is 

 said to be nearly 2000 feet thick in three mountains in Rosshire — Coul- 

 more, Coulbeg, and Suilvein ; and it is reported to be several thousand 

 feet in thickness in Caithness and other places, and to consist of both 

 red and gray grits. So far it resembles the Tyrone brownstone, and the 

 lower rocks of the Dingle district also, which are of Silurian age. 



In 1855 I saw a little of the Old Red Sandstone on the north side of 

 the valley of the Forth and Clyde, in Scotland. I believe the conglo- 

 merate at Callander to be the base of the Old Red of the Carboniferous 

 formation, and the equivalent of that rock in Ireland. 



From what I have just stated, it will be understood that there is a 

 strong conviction on my mind that the Old Red Sandstone of modern 



