Vol. 70.] OJftDOVICLAJSf AND SILURIAN OF LOUGH NAFOOLV. 105 



The published map of the distriet shows a stretch of tuffs 

 bounding the southern shore of Lough Nafooey, with a thick mass 

 of felsite on the south, which is followed by Salrock Beds (Ludlow), 

 and these by Upper Llandovery deposits. As will be seen later, 

 the beds mapped as tuff are in the main the basement-beds of 

 the immense series of grits and conglomerates known now as the 

 Mweelrea Grits, which compose the Formnamore plateau on the 

 north, and the lower part of which has been shown by Mr. Maufe 

 and Mr. Carruthers to be of Llandeilo age. The felsite of the 

 Survey is a thick series of spilite-flows, while the so-called ' Ludlow 

 Beds ' are in reality the lower members of the Llandovery Series. 



A list of the other papers bearing upon the geology of the 

 Kilbride district will be found in our paper on that area, 1 and, 

 as the list for the Lough Nafooey district would be identical, 

 it will not be repeated. Brief reports by the Committee of the 

 British Association appointed to investigate the igneous and 

 associated rocks of the Glensaul and Lough Nafooey districts 

 were read at the Dundee meeting in 1912, and at the Birmingham 

 meeting in 1913. 



The incompleteness of the 6-inch Ordnance Survey maps forms a 

 serious hindrance to satisfactory mapping in this part of Ireland. 



II. The Sedimentary a:nd Volcanic Arexio Bocks. 



As in the Kilbride area, the Arenig rocks are principally of 

 igneous origin, consisting of a great thickness of spilite-lava 

 associated with fine tuff and coarse breccia, and with a scanty 

 development of ordinary sediments. They occupy a strip of 

 country from a third to half a mile wide, which extends along 

 the northern part of the area, between the Mweelrea Grits on the 

 north and the Silurian strata on the south. 



(a) The Spilites (Pillow-Lavas). 



The spilite -lavas predominate over the fragmental rocks far 

 more in the Lough Nafooey area than in that of Kilbride. They 

 form the top of the ridge at Bencorragh, and extend for about half 

 a mile to the east and to the west of the summit of the hill ; while 

 they are exposed almost continuously in the lower ground on the 

 north from end to end of the area. They are in the main dark-green 

 line-grained rocks, and are often highly amygdaloidal, the amyg- 

 dules frequently consisting of epidote. Their most characteristic 

 feature, however, is the pillow- structure, which, while occurring 

 at numerous points, is particularly well-marked at the top of 

 Bencorragh. The arrangement of the vesicles in rows concentric 

 with the margin is not very general. As in the Kilbride area, the 

 spilite is frequently associated with abundant chert, which occupies 

 the interstices between the pillows. This is particularly well seen in 



1 Q. J.G. S. vol. lxviii (1912) p. 76. 



