488 Proceedings of the Royal Irish Academy. 



questionably to belong to it. These rocks, as previously mentioned, 

 lie nnconformably on the Arenig rocks to the north, and on the Cambro- 

 Silurians at Pomeroy and Lisbellaw. To the eastward, near Pomeroy, 

 and to the westward, in the Topped Mountain district, these rocks are 

 usually of a red colour and very arenaceous ; but S.E. of Six-mile-Cross, 

 where they appear to be best developed, there are, below, red arenaceous 

 rocks with subordinate limestones and shales ; above, there are green 

 sandstone and shales, with a few limestones ; while higher up there 

 seem to be red arenaceous rocks. This, as will hereafter appear, is 

 very similar to the section in the Ballaghaderreen district, Co. Mayo. 



Further westward the rocks of this basin were again shifted or 

 "heaved" southward by the faults of the Upper Lough Erne and 

 parallel valleys, they appearing from under the Carboniferous S.E. of 

 Lough Allen, near Drumshambo (Co. Leitrim). They are cut off on 

 the westward by a down-throw to the west ; but they shortly again 

 appear east of Lough Key and in the Curlew Mountain district, the 

 rise of ground from Lough Key, past Lough Gara, to the country N-W. 

 of Ballaghaderreen (Counties Roscommon, Sligo, and JS'.E. Mayo). 

 In the Curlew Mountains the base of these rocks is not exposed ; but 

 to the westward, between Ballaghaderreen, it is so, they resting un- 

 conformably on metamorphic rocks, either of Cambro- Silurian or 

 Cambrian age, as mentioned in the Paper " On some supposed Cam- 

 brian Pocks in the Co. Tyrone and N.E. Mayo." 



The mass of the rocks of the Curlew Mountain district belongs to 

 the red arenaceous or "Lower Old Red Sandstone" type, except per- 

 haps in the low country IS". "W". of Ballaghaderreen, where below and 

 above they are of this type, but between they are of the "argillaceous" 

 type, as more fully mentioned hereafter. All these rocks are evi- 

 dently detached portions of the basin, other portions of which are 

 found further south-westward, in the neighbourhood of Clew Bay, as 

 in the Croagh Moyle, Mulrany, Clare Island, and Louisburgh districts. 

 The rocks in these places, for the most part, are of the red arenaceous 

 type ; although in Clare Island and near Louisburgh there are greater 

 or less thicknesses of red argillaceous rocks. All these rocks from 

 Ballaghaderreen westward lie unconformably on metamorphic rocks, 

 which are either Cambro- Silurian, or passage rocks into the Cambrian 

 ("Arenig group," Upper Cambrian). 



Immediately south and south-east of Louisburgh, at Creggaunbaun, 

 and thereabouts, the Silurian rocks are, for the most part, metamor- 

 phosed, those that are unaltered belonging to the "argillaceous" type. 

 This is remarkable, because to the south thereof, in the Mweelrea dis- 

 trict, the rocks are principally of the red arenaceous type. The 

 Mweelrea district is a portion of the large Silurian area that extends 

 from Loughs Mask and Corrib, on the east, to the Atlantic, on the 

 west, in which the rocks of both types are intermingled. This area 

 occupies portions of S.W. Mayo and N.W. Galway. 



In the counties of Cork and Kerry are the rocks belonging to the 

 western extension of Gleikie's " Welsh lake basin." Their base is said 



