Kin AH AN — EiiriteSf or Basic Fektones of Sihirian Age. 341 



exhibiting the inteiiamellar structure described in the previous rocks. 

 Most of the crystals contain numerous enclosures, some of which are 

 recognizable as chlorite and opacite. Alteration appears to have 

 taken place extensively through the mass ; secondary products, some 

 of which appear to be calcite, occur, filling up the interstices. 



At Loughnafooey and the adjoining country, both in Galway and in 

 Mayo, are large tracts of eurites and their associated tuffs, partly in- 

 trusive, but generally interbedded. The specimen examined is from 

 one of the intrusive portions, and has a specific gravity of 2*62. 

 The ground-mass or matrix of this rock is very slightly granular, and 

 through it are scattered fragments of quartz, as also rounded portions 

 of a waxy-looking feldspar, probably triclinic ; in some of these an 

 effervescence takes place, when the specimen is treated with hydro- 

 chloric acid, probably due to films of calcite in cracks through the 

 minerals. Small earthy portions, having the form of orthoclase, 

 occur through the mass, as also fragments of a dark-green mineral. 



Mieroscopically, the ground-mass is cryptocrystalKne, and through it 

 occur fragments and crystals of quartz, feldspar, and microlithic por- 

 tions of hornblende. Ferrite is sparingly represented ; opacite occurs 

 plentifully, particularly round the edges of the quartz. The feld- 

 spars are, for the most part, more or less decomposed, but triclinic 

 twin structure is still apparent in many ; in others there is left but 

 an outer shell, the whole interior being replaced by an earthy, almost 

 impellucid, substance. In places there are fragments of calcite ; in 

 one case, a fragment of what was apparently once feldspar is now 

 represented by calcite. Apatite is recognizable in long acicular crys- 

 tals in some of the more pellucid feldspars. Some of the feldspars 

 show the interlamellar structure before described. The fragments of 

 quartz are clear, and contain some large stone enclosures and glass 

 cavities. 



In the Silurians north of Killary Harbour, at Bundorragha, and 

 to the west and east thereof, there are thick interbedded masses of 

 eurites, from one of which the specimen now to be described was 

 taken, the rock having a specific gravity of 2-66. There are some re- 

 markable differences between this rock and that next to be mentioned ; 

 as, although both are from the same neighbourhood, there is in the 

 latter a considerable quantity of carbonate of iron, although at the 

 same time its specific gravity (2-60) seems to be lower. The 

 ground-mass is a dense purple-brown matrix, having a sKghtly 

 resinous lustre, through which are scattered crystalline particles, 

 which, though tolerably abundant, are relatively small, and consist of 

 quartz and feldspar, the latter mostly having a pearly lustre ; some 

 present a structure that is very like that of labradorite. In thin 

 sections the ground-mass appears brown and fine-grained ; and scat- 

 tered through it are some comparatively large crystals and patches of 

 quartz, containing stone enclosures, as also numerous small patches 

 of clouded feldspar. 



MicroscoincaUy , the ground-mass might be described as a micro- 



