96 Proceedings of the Royal Irinh Academy. 



is found to be due to systems of minute gas bubbles, while the oblique 

 lining seems to be due to lines of shading caused by transparent and 

 translucent layers. 



In this mineral there are numerous inlying minerals or mineral 

 secretions, the most conspicuous being blebs of quartz which usually 

 have translucent nuclei, but a few have similar characters to the blebs 

 of quartz characteristic of the Elvanytes — other minerals observed were 

 amphibole, mica, pyrite, and magnetite. In fig. 3, PL 7, is shown a 

 bunch of three crystals of amphibole associated with a flake of mica. 

 In the associated orthoclase was observed a faint lining parallel to the 

 longer axis of the amphibole crystals, but the most conspicuous struc- 

 ture is that of the lines due to the systems of minute bubbles, some- 

 what similar to those mentioned when describing fig. 2, PL 7. In the 

 neighbourhood of the amphibole crystals, are many foreign particles 

 scattered about, some being roundish, but a few having irregular crys- 

 talline forms, all have translucent centres but some are siuTounded by 

 crystalline rims of quartz. A remarkable crystalline mass observed in 

 the orthoclase is represented in fig. 4, Plate 7. The dull grey por- 

 tions are slightly iridescent and of similar appearance to the dull white 

 •felspar hereafter to be described. In the orthoclase are also subvi- 

 treous, opaque black crystals ; two of which are represented in fig. 

 2, PL 7. 



Under a power of 238 some of the minute opaque specks appear to 

 be magnetite while others are probably pyrite (fig. 5, PL 7). 



In the smaller crystals and masses of the fiesh-coloured orthoclase, 

 the structure appears to be more regular ; as we find the crystals 

 crossed by wavy lines (fig. 6, PL 7) v.hich under a higher power (238) 

 are found to be due to clouds of minute gas bubbles which lie in irre- 

 gular, more or less parallel planes. These smaller crystals of the 

 orthoclase seem to contain only a few imbedded minerals principally 

 blebs of quartz and pyrite (?). 



Bull ivhite felspar (Adularia?). — This mineral in the Knockanavoddy 

 granite seems generally to occur massive ; there are, however, in places 

 well developed crystals, sometimes of considerable size. Under the low 

 power (42) this mineral seems to be irregular in its structure. Some 

 masses are nearly opaque, but in them, irregularly scattered, are 

 namerous iridescent specks, while in some are translucent portions, 

 seemingly of a triclinic felspar, as they transmit ribands of light (fig. 

 7, PL 7). In a crystalline mass (fig. 8, PL 7) was detected a lined 

 structure ; under polarized light, opaque lines, more or less regular, 

 alternating with subtranslucent iridescent broken lines. With a high 

 power (386) the cause of the lining is very obscure, as the constituents 

 are not found to be in layers. An opaque constituent, however, has a 

 tendency to lie in long irregular patches, parallel to the lining, while 

 the iridescent spots fonn groups in short lines. 



The peculiar appearance of some of the masses of felspar appears 

 to be due to opaque, subtranslucent and translucent particles that are 

 irregularly associated together. In one mass was nbsen'cd what seems 



