132 ME. GARDINEE AND PROF. REYNOLDS ON THE [May I909, 



Dole rites. — The dolerites are more numerous than the horn- 

 blende-lamprophyres, and occur at the following spots: — 



(1) In the bed of Stream G, about a quarter of a mile west -north- 



west of Gortbunacullin, two exposures of dark-green dolerite 

 are seen, one about 110 yards and the other about 60 yards 

 in length. The easternmost mass (78) has felsite exposed all 

 round it, and though felsite is only exposed on one side of the 

 other it seems probable that the two are parts of one and 

 the same intrusion in the felsite. The dolerite, being far more 

 easily eroded than the felsite, has here determined the course 

 of the stream. 



(2) Two exposures (86), no doubt forming parts of one mass of 



very fresh amygdaloidal olivine-dolerite, occur associated with 

 the basal Carboniferous sandstone in the lower part of Stream 

 G, half a mile north-north-west of Srah Bridge. The dolerite 

 appears to follow the bedding of the sandstone, and may be 

 intruded between it and the underlying red felsite, in which 

 case its age would be post-Carboniferous. 



The uppermost 2 inches of the dolerite are not amygdaloidal, 

 and are jointed much as is the associated sandstone. The 

 rest of the rock, however, is very amygdaloidal. A thickness 

 of some 2 feet is seen in a small vertical cliff on the southern 

 side of the stream, and the rock forms the floor of the stream 

 for some 40 yards. Both this and the dolerite-mass previously 

 described are characterized by a rough polygonal jointing on 

 a large scale, which is well seen in both cases in the bed of 

 the stream. 



(3) Very fresh olivine-dolerite (136) occurs in a field, a third of 



a mile west-north-west of Srah Bridge. Nothing is seen of the 

 neighbouring rocks but the line of junction of the Carboni- 

 ferous beds, and the red felsite probably passes quite close to 

 this point. 



(4) A similar mass of dolerite (143) is seen, not actually in place, 



but undoubtedly occurring at the spot, by the side of the road 

 which leaves the main road a quarter of a mile south of Srah 

 Bridge. Yery numerous blocks built into the walls occur 

 south-east of this point, between it and the main road, but 

 the rock has not been met with in place. 



It is not unlikely that these last three exposures may be 

 a single mass intruded along the line of junction of the 

 Ordovician and the Carboniferous rocks. 



(5) A somewhat different type of doleritic rock (54), which resembles 



the diabases of North Wales, occurs at the extreme north- 

 eastern limit of the Arenig Series exposed in the Mount Partry 

 and Monastery region, where the mass, which is surrounded by 

 tuff, has a visible length of about 50 yards. 



