28 



Proceedings of the Royal Irish Academy. 



shaped vesicles, which in the hand specimen give the rock somewhat the 

 appearance of an oolitic limestone (fig. 4). In nearly all cases these vesicles 



are lined with ealcite and filled in with radial 

 serpentine. A crack occurs in the rock which 

 is now filled in with ealcite, and the vesicles 

 adjacent to it are all filled with ealcite to the 

 exclusion of serpentine. The rock is an amyg- 

 daloidal diabase of a very basic type, and may 

 originally have been a limburgite, as no felspar 

 is observable. Another specimen examined from 

 the centre of this outcrop very much resembled 

 that previously described from the more northern 

 outcrop. It has numerous areas of notched 

 felspar, enclosing augite, apatite, and titaniferous 

 magnetite. The latter, which has undergone alteration into yellowish-white 

 leucoxene, is very abundant, and gives the rock a white speckled appearance. 

 Much augite occurs with the characteristic purple colour, and a little apatite. 

 This rock also shows the glomero-porphyritic structure referred to above, 

 the porphyritic aggregates being composed of olivine, which is now mainly 

 replaced by ealcite, with pyroxene showing decomposition into a greenish 

 mineral and a few irregular crystals of allanite. The matrix of the rock is 

 much decomposed, but was in all probability glassy. It now consists chiefly 

 of ealcite and chlorite. 



About half a mile due east of the summit of the hill another outcrop of 

 the intrusive rock was observed. It rises abruptly from the edge of the bog 

 to a height of 100 feet ou its eastern side, and abuts against the hill to the 

 It is almost circular in cross-section, and from its general appearance 

 is undoubtedly a small neck which has been choked with intrusive 

 material. The rock is compact and of a bluish colour, with a few cavities 

 containing ealcite. In section it is fine-grained, and has much ealcite 

 dispersed throughout. It has an altered glassy base with numerous micro- 

 lites of plagioclase felspar. Phenocrysta of hornblende occur as dark 

 granular pseudomorphs enclosing hexagonal crystals of apatite, some crystals 

 of which also occur scattered through the matrix. The felspar microlites. as 

 a rule, show no trace of flow-structure except where they approach the 

 phenocrysts, when they are seen to be arranged tangentially round them. 

 Much altered titaniferous magnetite occurs, particularly in the areas now 

 occupied by the hornblende pseudomorphs. 



The most easterly outcrop occurs a little over a mile in a south-easterly 

 direction from the summit of Croghan Hill. The ash here is continuous with 



