Joly — On the Minerals of the Dublin and Wicklotc Granite. 49 



markably fine crystals. Tourmaline, which most generally is part 

 of the immediate matrix of the more highly altered beryl, occurs 

 plentifully. Mixed with kaolinized matter, it is moulded often in 

 very large masses to the beryl, rarely penetrating the hexagons. I 

 possess, however, a specimen of beryl — from the Ballybetagh 

 quarry — in which a crystal of tourmaline, to all appearance, passes 

 through a well-formed hexagon from side to side. The beryl has 

 been altered, however, which, as we will see,- probably affords an 

 explanation. 



The beryls of Grlencullen present three types : normal crystals, 

 radiating crystals, and altered crystals. 



1. Normal Beryl. 



Pale apple-green; semi-transparent to translucent. Also yellow; 

 semi-transparent to translucent. Only faces definitely shown, base 

 and prism. The yellow varieties often present, on breaking the 

 crystals across, a core of green-coloured beryl. 



Specific gravity = 2 - 722 ; taken on a large green hexagon 

 weighing 86 grammes. 



Sections of these beryls, taken parallel to prism faces or to basal 

 faces, show numerous enclosures, vitreous with bubble or liquid 

 with bubble ; congregated in nebulae or arranged in strings. These, 

 taken at right angles to optic axis, show want of uniformity in 

 extinction between crossed nicols. There is a cross-hatched appear- 

 ance, as if the mineral was not crystallographically homogeneous 

 throughout. Des Oloizeau, on optical grounds, considered beryl as 

 probably possessing two optic axes close together. — Mineralogie, 

 p. 366, vol. i. 



These normal beryls cohabit with muscovite, which often closely 

 adheres over their prismatic faces. In size, crystals measuring 

 a couple of centimeters across the prism face are not uncommon. 

 Some years ago I took a crystal from the small opening in the 

 granite at Ballybetagh, which measured about 4'5 cm. across the 

 prism faces. 



Interpenetration by orthoclase is common in these crystals. I 

 have not seen any definitely penetrated by either mica or quartz. 



Beryl is not a phosphorescent mineral : if, however, some of 

 these crystals be heated in a dark room they will be found to 



