Joly — On the Minerals of the Dublin and Wickloiv Granite. 71 



twining line parallel to I, are also frequently met with. Occa- 

 sionally the crystals occur in radiating groups. When thus 

 arranged, it will be found that the basal faces have a sort of 

 symmetrical arrangement, being all oriented into planes perpen- 

 dicular to the plane of radiation, so that it is seen as radiating in 

 rectangular forms only. I may also observe, that so minute are 

 these crystals that they are freely contained and propped into 

 everj' conceivable position within the small thickness of the section. 

 This fact, coupled with the simultaneous focus of pinacoid, or 

 prism faces, on opposite sides of the plane of symmetry, renders 

 necessary considerable caution in deciding on the nature of the 

 forms in the field. 



The angles are generally sharp. Enclosures are rare ; generally 

 glass. Mutual interpenetration is very common. They present all 

 the appearance of having been formed antecedently to their felspa- 

 thio matrix. Colours are generally exquisite, but they will, of 

 course, vary with the thickness of the section in the field. Pale- 

 grey forms are not therefore to be put down necessarily as felspar — 

 they are not uncommon. The dichroiem is too feeble, seemingly, 

 in such small crystals to be of value in diagnosis. Branching 

 veins of a translucent greenish decomposition product cover the 

 faces in some cases. Iolite is known in many decomposition 

 forms. 



Figure 4, plate in., is a photograph showing a group of iolite 

 crystals sorted from those prepared for analysis, as described. They 

 are exceptionally large specimens. Enlargement, 18 diameters. 



Figure 3, plate in., more highly magnified (x 70 diameters), 

 shows iolite in situ. A polygonal form, slightly turned up, so as 

 to show the faces i - i and / conspicuously, as well as the basal face 

 0, occupies the centre of the field. The other forms are mostly 

 rectangular, parallel, more or less, to the face i - i. Some of these 

 show the i-i cleavage. They nearly all extinguish longitudinally. 

 The chequered appearance of the felspar is displayed over the 

 field. 



