KiNAHAN — On Jlicroiscopical Struct tire of Ilocl:s. 95 



mica and widely disseminated crystals of titanite. The dull white is 

 the most abundant felspar in the rock, but the flesh-coloured appears in 

 large crystals, often twins, and gives the porphyritic character to the 

 rock mass. Most of the black mica, the amphibole, and the pyrite, 

 occur associated together in nests, the quartz forming the skeleton of 

 the rock. Of this rock two slices were cut, one containing one of the large 

 flesh-coloured felspar crystals, the other being from a portion of the 

 rock showing its ordinary character. In the latter slide we find the 

 felspars are principally represented by the white variety, only one or 

 two small pieces of the flesh-coloured being present, while the green is- 

 scarcely represented, but in the other slide all the minerals mentioned 

 in the field list are represented. 



No perfect crystals of any of the felspars appear in the portions 

 of the rock from which the slices were cut, but the flesh-coloured seems 

 to be nearest perfection, and we know that in other portions of the rock 

 mass, perfect crystals of this kind of felspar can be procured. The 

 dull white and the green felspars seem to be jumbled together, the 

 white predominating. In the nests, containing pyrite amphibole and 

 black mica, these minerals seem to have crystallized out in the order in 

 which they are mentioned ; indeed, in places, the pyrite and amphibole 

 seem to have crystallized out prior to the felspars, as perfect crystals of 

 both occur in them, but the mica seems to have been formed sub- 

 sequently, as flakes of both black and white mica are found in places 

 margining the felspars. The mass of the quartz fills the vacancies left 

 after the other minerals were fonned, but blebs of quartz oceiu' in the 

 felspars. The titanite when it occurs is always in well fonned crystals. 



Flesh-coloured felspar, — This according to Haughton is orthoclase. 

 It seems, however, to be very irregularly constituted and to contain 

 many impurities. When we examine a slice of white orthoclase from 

 Ytterby, Sweden, for a specimen of which I am indebted to my col- 

 league, F. Eutley, F. G. Si, we find it to consist of nearly parallel 

 transparent and semitransparent lines (see fig. 1, PI. 7) alternating, 

 which under a higher power (238) shows the lining to be clue to sys- 

 tems and lines of minute gas bubbles. This structure is traversed 

 obKquely by lines of fracture, (?) and when examined with the polar- 

 izer the matrix of the mineral appears mottled in places with more or 

 less irregular specks and patches ; it is sometimes marked with irregu- 

 lar lines, and in some places exhibits a tendency to a structure parallel 

 to the lines of fracture (?). Under the low power (42) few included 

 minerals were observed, the most remarkable being small crystals of 

 pyrite (?) and minute blebs of quartz. 



In one of the slices of the Knockanavoddy granite, there is a large 

 twin crystal of flesh-coloured orthoclase, in which the junction of the 

 crystals is most marked. In the left hand crystal at the top there is a, 

 wavy oblique lining that does not appear in the rest of the portion 

 under examination, but the most conspicuous structures in both crystals 

 are irregular and irregularly placed lines rudely parallel or nearly so 

 to the junction of the twins, and this structure under a power of 296 



