286 Scientific Proceedings, Royal Dublin- Society. 



In the Curlew mountains, between Charlestown and Ballagha- 

 derreen, the base of the Silurians is still in dispute, while the 

 Silurians N.E. of Clew Bay, in Croaghmoyle and thereabout, are 

 principally conglomerates and breccias, which are easily distinguish- 

 able from the associated older rocks. 



At the south margin of the Louisburgh Silurian Basin, S.W. 

 of Clew Bay, there is a conglomerate ; and thereby the boundary 

 is indicated ; but to the N.E., where both the Ordovicians and the 

 Silurians are metamorphosed, the base of the latter is obscure. 



In the Gralway and Mayo Basin of Silurians the boundary has 

 different phases. To the north-eastward, in Slieve Par try, the 

 rocks, when principally massive conglomerates, are easily distin- 

 guished from the associated older strata ; but the north boundary 

 here and in Mweelrea is obscure, as there is no basal conglomerate, 

 while the Silurians and Ordovicians are very similar in their 

 composition and in their strike ; thus the boundary between them 

 would be difficult to trace, if it were not that along most of it 

 there is a bed of felstone (euryte), or a peculiar quartzy-rock ; but 

 where these are absent, as they are to the westward, the rocks of 

 Silurian and Ordovician age are scarcely distinguishable. The 

 older rocks south of the west portion of the basin are metamor- 

 phosed, and, consequently, the south boundary was easily traced : 

 otherwise it would have been difficult to make out, as only in a 

 few places are there small patches of the basal conglomerate, while 

 the older and later rocks have in general very similar strikes and 

 dips. 



In the Dingle promontory, Co. Kerry, the boundary between 

 the lowest group of the Silurians (Smerwick Beds), and the Ordovi- 

 cians {Anascaul Beds) is obscure, but here it is possible that the 

 Smerwick Beds may be the passage or " gap " rocks between the 

 Silurians and the Ordovicians, and that through them there may 

 be a regular sequence. 



In the Co. Waterford, at Knockmahon, a conglomerate lies 

 unconformably on the Ordovicians; but farther westward, at 

 Bunmahon Head, and in Ballydowane Bay, this conglomerate 

 appears as masses in the Ordovicians ; in the Commeragh moun- 

 tains, however, there are massive conglomerates lying uncon- 

 formably on the Ordovicians. 



