108 PROCEEDINGS OF SOCIETIES. 



with diffidence. It appears to me that the chief part of the Old Red 

 Sandstone of Herefordshire, described in " Siluria," p. 242, as 8000 or 

 10,000 feet thick, is a Silurian rock, and the equivalent of the brown 

 rocks between Trillick and Pomeroy, in Tyrone, and the lower rocks 

 about Dingle, in Kerry. In both places in Ireland the underlying fos- 

 siliferous Silurian rocks are conformable with the brown grits, which I 

 have in another place called brownstone. So they are in Brecknock 

 and Herefordshire, on the line from Llandeilo by Kingston to Aymestry . 

 In physical characters, too, they agree. For a few days last autumn I ex- 

 amined the country between Abergavenny and Brecon, in Wales, and 

 along the road for several miles south of the latter place I saw those 

 brown, gray, and green grits and slates, and the best eye could not dis- 

 tinguish them from the Dingle grits or slates in grain or colour. 



But it is stated that there is a regular passage from the Old Red 

 Sandstone of Herefordshire downwards into the Silurian, and upwards 

 into the Carboniferous system of South Wales. We have the passage 

 downwards from our brownstone into the Silurian fossiliferous bands ; 

 but we have no such passage as that described upwards in either the 

 Dingle or the Tyrone district, and it appears to me very doubtful that 

 such a passage exists even about the Yans of Brecon : for it must be 

 there, if anywhere. 



Sir Henry de la Beche seems to have recognised an unconformabi- 

 lity in this district ; for, in the " Memoir of the Geological Survey of 

 Great Britain," vol. i., p. 59, he says : — " On the north of the great 

 coal-fields of South Wales and its supporting limestone the upper part 

 of the Old Red Sandstone forms a range of lofty land, of which the Yans 

 of Brecon attain the highest elevation (2682 feet). From the small 

 angle of the dip the continuation of the beds forming the summits of the 

 Vans is only a few feet beneath the Carboniferous Limestone, near 

 Merthyr Tydvil." Again, at p. 60, he says : — " Proceeding towards 

 Carmarthen, not only do we appear to find a mingling of sand more, at 

 the same geological time, westward than eastward, but also an overlap of 

 the higher arenaceous and conglomerate series upon the lower and marly ac- 

 cumulations of the Old Red Sandstone ; the Carboniferous Limestone 

 and Coal-measures over the Silurian rocks." 



What is this overlap of the higher arenaceous and conglomerate se- 

 ries, and the rest, over the Silurian rocks ? It appears to me to be 

 this : — That the conglomerate of the Yans of Brecon, or its equivalent, 

 lies unconformably on the Silurian rocks near Llandeilo, and, of course, 

 also on the Old Red Sandstone of Brecknock and Herefordshire ; for, be 

 it remembered, that the two are conformable. 



This leads to the conclusion that this Old Red Sandstone is divisible 

 into two parts, the conglomerate and sandstone of the upper part at the 

 Yans of Brecon being the base of the Carboniferous system, and there 

 lying unconformably on the lower or supporting Old Red Sandstone, as 

 it does at Dingle in Kerry, and at Clogher in Tyrone. 



As well as Sir Henry de la Beche, already quoted, Professor Sedg- 

 wick appears to entertain the same views that I do — mine taken, as before, 



