Vol. 70.] ORDOYICIAX AXD SILUEIAX OF LOUGH XAFOOEY. 107 



(xreenaun in the Glensaul district. The he of the breccias them- 

 selves is not apparent, but the fact that they overlie the spilites on 

 the south is proved by the occurrence along part of their southern 

 border of red sandy shales and cherts dipping northwards at 70°. 

 In Two-Stream Valley these breccias and felsites are cut off on the 

 west by a fault, and for about a mile westwards, that is, as far 

 as the stream which is shown in the 6-inch Ordnance Survey map 

 as entering Lough Nafooey at the promontory half a mile west 

 of Red Island, only small patches of breccia occur associated with 

 the spilites and lying near the boundary of the Mweelrea Grits. 

 To the west of this stream, however, a large patch of felsitie 

 breccia is exposed; and a still larger patch of tuff, mainly of fine 

 grain, is seen round the hamlet of Curraghrevagh. 



These rocks are nearly all vertical, and strike more or less east 

 and west ; but on the steep north-western slope of Bencorragh a 

 band of breccia, 12 feet wide, associated with a bostonite-dyke, 

 occurs, striking in a north-north-easterly direction. This and 

 another smaller patch, half a mile east of the top of Bencorragh, 

 are the only breccias that we have found in the heart of the spilites. 



(2) The Shales, Cherts, and Fine Tuffs. 



At several points the breccias and spilites of the north-eastern part 

 of the area here described are associated with fine tuffs and cherts. 

 Thus, at a spot not far from the Finny Biver, the breccias include a 

 band of fine tuff and red chert dipping northwards at 82°; while the 

 shales and cherts near the line of junction of the felsite-mass, south- 

 west of these breccias, with the spilites, have already been mentioned. 

 On the western side of the fault which brings the breccia against 

 the spilite of Two-Stream Valley, red shales and cherts are seen 

 along two lines. The southern one begins at a waterfall where 

 12 feet of chert, shale, and grit are associated with a small felsite- 

 dyke, and are traceable westwards in the spilite for a third of a mile : 

 the strata being vertical throughout their whole length of outcrop. 

 The more northerly series of exposures is longer and far less con- 

 tinuous. The strata are first seen in the lower part of Two- 

 Stream Valley, where the united streams, immediately after their 

 junction, pass through a little gorge. Here the rocks are vertical, 

 and the succession from north to south is : — 



Thickness in feet. 



Eed chert 10 



Fine tuff 18 



Red chert 2 



Spilite. 



The relation of tuffs, cherts, and spilites at one locality on the 

 right bank is shown in the accompanying sketch (fig. 1, p. 106). 

 The cherts contain abundant evidence of the presence of very 

 small radiolaria. Dr. G. J. Hinde, F.R.S., who kindly confirmed 

 our opinion on this point, writes that 



' the large majority are simple spheres or ellipses, which would be placed 

 under Ceno$i)hxra or Cenellipsis ; there is an elongated form with rounded 



