Kin A HAN — On Mlcromopkal Structure of Bocks. 97 



to be small gas bubbles of the same class as those mentioned when 

 describing the flesh-coloured orthoclase, but in no other of the masses 

 could they be detected, and they do not appear to affect the structure 

 of the mineral. Very few inlying minerals were observed in this 

 felspar, the most marked being minute blebs of quartz, and black 

 specks that may be pyrite or magnetite. This felspar is evidently one 

 of the orthoclases, probably adularia. 



Greenish ivaxy fehpar (Oligoclase?). — This triclinic felspar is not 

 very well represented in either of the- slices that were cut fi'om the 

 specimen of the Knockanavoddy granite, as in one it scarcely appears, 

 while in the other only portions of crystals occur ; but in other places 

 in the neighbouring rock we know it was more frequent. This felspar 

 we suppose to be oligoclase, and we will, therefore, first describe the 

 appearance of oligoclase fi-om Ytterby, Sweden (F. Eutley). A slice of 

 the latter mineral under the low power (42) is found to have originally 

 consisted of regular lines of darkish and light colours, which are now 

 broken up by long but irregular spaces, that appear to have been due 

 to shrinkage fissures after the mineral was formed, while scattered 

 about are inlying crystals, blebs, and particles of quartz and small 

 opaque specks (fig. 9, PL 7). In fig. 10, PI. 7, is represented por- 

 tions of the green waxy felspar of the Knockanavoddy granite. In them 

 the riband of colour is well developed which gives a vivid change of 

 colour when the polarizer is turned, especially if viewed through a 

 plate of selenite ; at {a) are irregular flakes of mica, at (V) a crystal that 

 appears to be titanite, as it is similar to a large crystal, part of which, 

 with two inliers of the same mineral, is represented in fig. 1 1 , while 

 at (c) a portion of a roundish but irregular mass of felspar (orthoclase ?) 

 appears. When the riband at id') (fig. 10) is magnified (386) the 

 mineral is found to be in places full of minute cavities, some round, 

 others vermicular and sinuose, while the rest have iiTegular forms. In 

 places these occur in lines parallel to the riband, but the mass of them 

 cross it in clouds and, therefore, can have no connexion with the 

 structure of the rock as seen with the low power (42), but to the 

 clouds crossing the structure, may be due the faint transverse play of 

 colours. The oligoclase from Ytterby, when viewed under the high 

 power (386), is also found to be affected by similar cavities, but in it 

 they have a tendency to run in lines parallel to the riband. These 

 cavities appear to be due to minute gas bubbles attached to the sides 

 of minute vacancies, and these cavities in the Swedish specimens are 

 in general, but not always, nearly parallel to the riband, while in the 

 Knockanavoddy granite they run very irregularly. 



B.'^ Porpliyritic granite from Ballynahown (Gralway sheet 93). — This 

 rock, when examined in the field, seemed to consist of flesh-coloured 

 felspar, dull or olive green felspar, quartz, black mica, amphibole, 

 pyrite, and white mica. The flesh-coloured felspar appears principally 

 in large crystals, often twins, which are imbedded in a matrix princi- 

 pally made up of the green felspar and the quartz. Most of the 

 amphibole, black mica, and pyrite are associated together in nests, 



