138 ME. GARDINER AND PROF. REYNOLDS ON THE [May I909, 



The rock (143), seen along the turning which leaves the main 

 road a quarter of a mile south of Srah Bridge, is of the same general 

 character, but somewhat finer graiued. A vesicular dolerite, from 

 the lower part of Stream G near (86), differs from the other dolerites 

 of that locality, in having its augite and olivine mainly replaced by 

 serpentine. 



In addition to these fresh olivine-dolerites, an allied rock (51) 

 occurs at a point about a quarter of a mile north of the Monastery, 

 differing from the one just described in its weathered condition and 

 in being devoid of olivine. In a hand-specimen the freshest 

 examples of this rock are dark green and show numerous augites 

 and colourless felspars. In one variety, however, the felspar- 

 crystals are stained red. In section the rock is seen to be much 

 decomposed. Though some fresh unaltered augite is present, most 

 of the augite is chloritized. The felspars, which sometimes show 

 twinning, are, as a rule, replaced by secondary quartz and calcite. 

 Ilmenite and apatite are plentiful. The specific gravity of this 

 rock is 2-79. 



(d) The Tuffs and Breccias. 



Pour principal rock-types may be readily recognized : — 



(1) The fine gritty tuffs. 



(2) The fine non-gritty tuffs. 



(3) The limestone-breccias. 



(4) The coarse fel site-breccias or agglomerates. 



(1) The fine gritty tuffs. — These rocks, which pass gradually 

 on the one hand into pure grits, and on the other into tuffs free from 

 gritty material, are undoubtedly the type of tuff most prevalent in 

 the district. They are largely developed along the whole of the 

 western outcrop of the red felsite, but are especially well seen in the 

 neighbourhood of Gortbunacullin, where they have yielded many 

 fossils. In hand-specimens they are usually red or brownish rocks 

 of medium to fine grain, showing much quartz and many felsite- 

 fragments. They often effervesce rather strongly with acid. In 

 section the abundant angular quartz-grains are seen to be mingled 

 with closely packed felsite-lapilli of several types, and occasional 

 andesitic fragments, as, for example, in (167), which shows lapilli of 

 a vesicular andesite or basalt containing fairly fresh augites. All 

 these are united by a calcareous cement the proportion of which 

 is variable (see PI. Y, fig. 5). Eed iron-oxide is sometimes abundant, 

 forming a stain pervading the section. Magnetite is also rather 

 common. Occasionally a rock is met with (122) in which felspar- 

 crystals play a prominent part. 



(2) The fine non-gritty tuffs. 1 — These rocks, some of which 

 are of Llandeilo, others of Arenig age, are principally met with in 



1 It was sometimes very difficult to distinguish between certain of the tuffs 

 and the finer types of felsites. Mr. A. Harker, F.R.S., has kindly helped us, 

 and we desire to tender him our sincere thanks. 



