112 



Proceedings of the Royal Irish Academy. 



The Old Bed Sandstone. 



The Old Eed Sandstone or " Dingle beds " of the Pomeroy district 

 were not examined in any detail ; but during the attempt to discover 

 the boundary of the Lower Palfeozoic rocks, sufficient was seen to 

 render it evident that the two series are separated by a strong 

 unconformity, and that the Dingle beds overlap the Silurian on to 

 the ancient hornblendic rocks. The basal Dingle beds do not follow 

 the line of the colour-change marked upon the old one-inch Survey 

 map of 1877, but instead come on at the various exposures indicated 

 upon that map as conglomeratic Bala beds ; and accordingly the 

 failure of later visitors to the district to find evidence of unconformity 

 along the line mapped is not to be wondered at. The basal Dingle 

 beds of our mapping are usually massive or flaggy sandstones ; but 

 they include many beds of coarse, rubbly grits, and conglomerate or 

 other resistant rocks, and contain many pebbles of quartz. Among 

 these pebbles we were struck by the abundance of subangular or 

 even angular cherts and lydian stones, such as might come from the 

 Arenig or Llandeilo cherts of South Scotland. AYe also noticed 

 several decomposed fragments of a hornblende granite, which we think 

 may be the Eardahessiagh rock, and with it many more or less decayed 

 pieces of Ordovician and Silurian sediments with obscure fossils. 



The basement beds of conglomerate were never seen ; and no 

 section giving a full view of the unconformity could be found. The 

 boundary of the series as indicated upon the map is really a line 

 drawn to pass between certain localities; and if the smooth-flowing 

 outlines indicated are a trifle artificial, we can only say that they are 

 not claimed as more than a diagrammatic representation of the truth. 

 The dip of the Dingle beds is less constant in direction than that of 

 the Lower Palaeozoic rocks ; but it does not differ greatly from the 

 latter in magnitude; and it would seem that, though less yielding in 

 their behaviour, the Dingle beds have been involved in nearly all the 

 earth -movements which have contorted the lower series. Since, 

 however, about Slievebane, the red and green sandstones of the Dingle 

 beds rest upon the Hornblendic Series, while at Little River, they 

 transgress from the Killey Bridge beds on to the Tirnaskea beds, and 

 at Corrycroar rest upon the Corrycroar group and Slate Quarry beds 

 of the Little River group, there appears to be sufficient proof of the 

 strong and irregular folding, accompanying a maiked unconformity m 

 pre-Dingie times. 



