342 Proceedings of the Royal Irish Academy. 



felsitic base, with numerous minute microcrystalline particles scat- 

 tered through it. Through this matrix are scattered crystals and 

 fragments of feldspar, both orthoclase and triclinic ; the latter being 

 the more abundant, though the fragments are, as a rule, smaller than 

 those of orthoclase. Many of the fragments are clouded, some greatly 

 altered, so that it is difficult to say whether they are orthoclase or 

 triclinic ; in a few cases, both varieties appear to occur in the same 

 particle. The interlamellar structure before described is to be ob- 

 served in some. The quartz crystals contain stone enclosures, while 

 chlorite and opacite occur in the ground-mass ; the former were also 

 observed in some of the quartz crystals. Through the ground-mass 

 there are portions that seem to have been geodes ; on the sides of 

 these are microcrystalline particles of feldspar and quartz ; the inter- 

 stices have since been filled with some secondary product. 



Another specimen, from the bedded masses near Bundorragha, 

 has a specific gravity of 2-60. It consists of a brown felsitic ground- 

 mass, having a finely-granular texture, through which are scattered 

 blebs of quartz, and also crystals of feldspar having a pearly lustre, 

 with patches of a brownish mineral (carbonate of iron) which effer- 

 vesces copiously on the application of hydrochloric acid ; effervescence 

 also takes place in minute specks through the ground-mass, particu- 

 larly at some little green patches, as if carbonate of iron were very 

 prevalent in the rock. The slide shows the ground-mass to be com- 

 posed of a compact brown matrix, through which are numerous 

 minute fragments of a fibrous-looking mineral, a few large fragments 

 of quartz, a clear feldspar with numerous irregular lines through it, 

 also a clouded feldspar, and a brown mineral (iron). There are also 

 minute cavities round which are incrustations. 



Microscopically, the specimen consists of a cryptocrystalline matrix, 

 alteration-products occurring plentifully ; few recognizable minerals 

 occur ; but there is a cloudy altered feldspar, which appears to be 

 orthoclasic, and also a clear, homogeneous, triclinic feldspar with 

 numerous irregular lines passing through it, along which alteration 

 has taken place ; quartz is present in small quantities, and scales of 

 iron carbonate. 



