340 Proceedings of the Royal Irish Academy. 



mass or matrix shows a minutely granular texture ; in it are enclosed 

 small rounded blebs of quartz, with crystals (some twins of the 

 Carlsbad type) of orthoclase and of triclinic feldspars ; some of the 

 latter are like labradorite. In thin sections the feldspars are 

 generally of an opaque white, and are apparently much altered. 

 There are also small fragments of a greenish mineral, having their 

 longer axes lying in one general direction, giving the slide a foliated 

 appearance. In the specimen a few crystals of a green earthy 

 mineral are visible, probably an altered hornblende. 



Microscopically , the ground-mass of this rock is cryptocrystalline ; 

 that is, though evidently crystalline, the component minerals are 

 not recognisable. Scattered plentifully through it is a fine opaque 

 dusty-looking mineral (opacite), and a green chloritic mineral 

 (viiidite), which, together, give the slice a banded or fluidal struc- 

 ture, circling round and tailing out behind the larger ciystals. 

 Scattered through the ground-mass are the macroscopic crystals of 

 feldspar, which, in general, have not sharply-defined angles. In 

 many cases, under ordinary light, they have very much the same 

 structure as the surrounding matrix, being scarcely distinguishable 

 from it, except on account of a clearer margin along the boundary, 

 while in other cases they are more impellucid than the matrix ; 

 but under polarized light they are plainly discernible ; as under 

 crossed jSTicols the matrix presents a speckled grey appearance, 

 while the feldspars are either of a uniform light tint, or else 

 exhibit the banded structure, with iridescent specks, as detailed 

 in the description of the Lough Guitane rock. The quartz is clear, 

 but contains bays and enclosures of the matrix (stone enclosures). 



At Clogher Head, Dingle Promontory, the eurites appear to be inter- 

 stratified with rocks containing typical Silurian fossils. The specimen 

 taken thence is a pink-brown-coloured eurite, having a specific 

 gravity of 2-71. The ground-mass is a pink-brown feldspartic matrix, 

 through which are scattered small crystals of orthoclase (some Carls- 

 bad twins), and triclinic feldspar (probably both labradorite and 

 oligoclase). These ciystals are mostly long and slender; but some 

 are broken, apparently along a twin plane, and the ground-mass is 

 forced in between the fi'actured portions. In thin sections there is a 

 banded appearance in the slice, some of these bands being more 

 impellucid than the rest ; they lie in one general direction. On 

 treating the specimen with hydrochloric acid, effervescence takes 

 place around some of the triclinic feldspars, also along lines or cracks, 

 and at little specks through the ground-mass. 



Microscopically , the ground-mass is composed of a microf elsitic base, 

 containing small crystalline particles of quartz, feldspar, viridite, 

 and opacite, exhibiting a slight fluidal structm-e. Thi-ough this 

 microfelsitie base are bands having a microcrystalline structure, com- 

 posed of quartz and feldspar ; and through this ground-mass are 

 developed porphyritically crystals of orthoclase feldspars, more or less 

 clouded, with twinning, some being broken. There arc also feldspars 



