Joly — On the Minerals of the Dublin and Wicklow Granite. 65 



Briefly summing up the results of these various observations, 

 it appears, with great probability, that the heterogeneous crystals 

 were primarily composed entirely of beryl : subjected secondarily 

 to reaction with a potash felspar in a state of hot solution, they 

 were partially replaced, and that to very different degrees ; that 

 the primal seat of this reaction is, in general, traceable to one 

 region of the crystal, now the implanted surface, in which direction 

 the replacement is most complete, the original structure of the 

 crystal being often completely broken down ; and that this reac- 

 tion, being confined between beryl and felspar, allowed of the 

 hexagonal form being preserved within the schorlifferous matrix, 

 the result being a variable mixture of felspar and beryl pseudo- 

 morphous after beryl. The felspar so mixed with the beryl is 

 orthoclase, containing a mineral in general foreign to Irish rocks, 

 iolite, and, further, containing, there is reason to believe, glu- 

 cina ; but this question is not gone into in the present Paper. 



Subsequently, and probably as the effects of hydration, the 

 mixture has been kaolinized to variable extents ; and, as a tertiary 

 alteration also, a chloritic mineral has been formed through the 

 beryl. 



The beryls of Grlencullen often radiate in beautiful conical 

 bunches, with a completeness and regularity not noticed in de- 

 scriptive mineralogy. 



These are often of delicate green and yellow hues ; and such, 

 in common with crystals of normal habit, lose nearly all colour 

 after being exposed for a short time to a temperature of about 

 350° C. 



The Iolite of Glencullen. 



I now turn to the consideration of the very minute crystals 

 developed through the felspar, and absent from those portions of 

 the sections composed of beryl. 



The crystals appeared in two types : — a wide polygonal form, 

 often with twelve edges, extinguishing along two edges situated at 

 right angles to each other on the polygon ; a rectangular elon- 

 gated form, extinguishing most generally along the edges, but 

 often at variable angles with the edges. Foliation in thin plates 



