Smyth — On a Variety of Finite occurring at Balli/corus, Co. Dublin. 195 



The mineral under discussion, then, is evidently a pinite. It seems to me 

 unnecessary to invent any more definite term. 



Since killinite, another variety of pinite, occurs, also near the granite 

 margin, at Killiney, a few miles from Ballycorus, we must distinguish it from 

 the present mineral. It differs most distinctly (1) in being pseudomorphous 

 after spodumene, (2) in being fusible before the blowpipe.^ 



Origin and Relations. 



A number of thin slices of granite in various stages of alteration were 

 prepared. One fresh-looking specimen contains abundant allotriomorphic 

 microcline, with strong, typical cross-hatching, only slightly cloudy on certain 

 laminae, and often iucluding idioniorphic crystals of the other felspars. There 

 is much albite, mostly speckled over with alteration products. What appears 

 to be orthoclase is much altered, the secondary material usually occupying all 

 the centre of the crystal, and agreeing in appearance with the pinite. The 

 quartz is in groups of grains, and there is a fair amount of muscovite. 

 Cataclastic structures are evident. The quartz is in places finely granular, 

 and the mica bent and broken. In a shear zone the latter seems to be groixnd 

 to a fine granular paste, which is doubtfully distinguishable in places from 

 the pinite. A little galena is present. 



Slices containing a considerable amount of pinite appear to show that it 

 originates, at least chiefly, from felspar. The latter often occurs like an 

 archipelago surrounded by a sea of the alteration product, the islands being 

 optically parallel. In this material the felspars have rather indefinite optical 

 characters. Considerable fragments may appear untwiuned, but a hio-her 

 magnification often reveals the microcline structure on a minute scale in 

 spots, mostly at the edges. What appears to be secondary lamellar twinning 

 is often seen. In several cases there cross such felspars broad bands, 

 suggesting shearing planes, distinguished by more intense alteration. Quartz 

 is generally abundant, often showing strain shadows, which are sometimes 

 arranged fanwise. 



A slice made from a piece of rock having a fairly uniform green colour, but 

 a dull lustre, was found to consist of pinite with abundant, but small, ragged 

 quartz grains, a very few similar fragments of felspar, and still fewer of 

 muscovite. 



A remarkable relation between quartz and pinite is obser\'ed in some 



' Tlios. Taylor, " Account of a New Mmeral Substance discovered ac Killiney, in the 

 vicinity of Dublin." Trans. Roy. Ir. Acad., vol. xiii, p. 61. 1818. 

 T. Thompson, " Outlines of Mineralogy," vol. i, p. 330. 1836. 



