Vol. 70.] ORDOYICIAX AXD SILUBIAX OF LOUGH XAFOOEV. 113 



(c) The Lime-Bostoriite. 



As in the 'Kilbride area, this rock forms a sill at the base of the 

 Llandovery Beds, whether they rest on the Arenig rocks, as they do 

 throughout by far the greater part of the area, or on the Mweelrea 

 Grits (Llandeilo), as is the case at the extreme western end. In 

 the Lough Nafooey area, in addition to the main intrusive sill, 

 there are three minor dykes of this rock occurring in the Arenig 

 spilite. 



The bostonite is very constant in character along its whole out- 

 crop, being very fine-grained, of a dark purple colour, and without 

 obvious phenocrysts. While as a rule devoid of vesicles, it some- 

 times becomes highly amygdaloidal. The thickness varies con- 

 siderably, but is on the whole far less than in the Kilbride 

 area, being frequently only some 30 feet. There is progressive 

 diminution in thickness as the rock is followed from east to west, 

 and, both north-east and north-west of the top of Curraghrevagh 

 occur tracts where the bostonite is absent. On the other hand, it 

 locally thickens to 80 feet, as in the bed of one of the streams 

 flowing down from the cirque north of Benbeg. These streams 

 afford an interesting and puzzling series of sections, and merit a 

 somewhat detailed description. 



The westernmost stream (A on the map, PL XYII) shows no 

 exposure below the conglomeratic base of the red sandstone. 



The main stream (B) shows the succession of exposures as seen 

 in fig. 3, p. 114. There are two cascades the lower of which is over 

 bostonite, resting upon an ill-defined gritty rock, and overlain by red 

 sandstone with a conglomeratic base. Above this red sandstone 

 comes a second band of bostonite overlain bv a thick conglomerate, 

 with patches of shale at the base. This conglomerate forms a 

 relatively level tract of which the footpath crossing the stream 

 takes advantage, and above it is a thick series of red sandstones 

 over which the main cascade falls. The occurrence of two bands 

 of bostonite might be explained as a ease of the bifurcation of an 

 intrusion, but as each band of bostonite is succeeded by con- 

 glomerate and red sandstone, it seems more probable that the 

 repetition is due to a fault (see fig. 3). 



Stream C. — Here red sandstone without any conglomerate at 

 the base rests upon bostonite, below which is an ill-defined gritty 

 rock succeeded by a small dolerite-dvke. In the map (PL XVII) 

 these ill-defined gritty rocks in streams B and C are coloured as 

 spilite. 



Finally, stream D, for some 40 yards above its junction with 

 stream C, exposes felsite : then, after a gap, comes a curious rock 

 which at first sight resembles a conglomerate. The 'pebbles' arc 

 however, all red felsite, and it appears that the entire rock is really 

 a much-crushed and weathered felsite. The upper part includes 

 patches of very pyritous black shale and black chert. A band of 

 bostonite about 80 feet thick follows, succeeded by red sandstone. 

 A short distance east of this stream a fault brings on spilite. 

 To this fault may be attributed, not only the crushed character of 



