Jolt — On the Minerals of the Dublin and Wicklow Granite. 69 



The characteristics ultimately determined are recorded in the 

 annexed Table, in the right-hand column. The characteristics 

 of Iolite, the Cordierite of the French school, appear in the left. 

 Taken collectively the evidence is, I think, irresistible that these 

 small crystals are iolite. 



Figure 1 on the Table, right-hand column, depicts the basal 

 face of the crystal. It shows foliated habit and twin-line, or trace 

 of composition plane I. 



The angles I had in the first instance determined as the mean 

 of many measurements were so close to 150° each, that I decided 

 to enter them as such. This gives nearly the same values as Dana 

 and others record: i" a /is 120° and 60°, and the secondary face 

 in the zone n becomes nearly * - 3. 



Extinction is along i - I and i - i, shown by the arrow-head. 

 The face i - 3 is often absent, and always small. Fractured corners, 

 as in fig. 1, are common. The cleavage is then well seen. 

 Figure 2 is elevation of zone n. Extinction, as shown. 

 Figure 3 is section on i - £, showing cleavage. 

 Figure 4 is end-elevation of zone n. 



Figure 5 is section on i - l. Cleavage very obscure, or absent. 

 Figure 6 is a hemihedral form respecting i -3 ; not very un- 

 common. 



Looking down the column, I need only observe that the colour 

 is only seen when a large number of crystals are superimposed, as 

 in a narrow test-tube, and viewed by transmitted light. 



The hardness was only very approximately determined by press- 

 ing a number of the crystals into the end of a lead wire, and then, 

 using the wire as a handle, proceeding as usual. 



The fusibility was determined to be about the same as iolite 

 by comparison with an authenticated specimen on the meldometer. 

 It is perhaps a little higher than that of the specimen used ; nor 

 did the known iolite whiten to any great extent. The loss of 

 transparency experienced by iolite on fusing is however well known. 

 Lime is generally present in this mineral : its absence might 

 account for exceptional behaviour in this respect. 



An analysis, made with Mr. Early's assistance, is. added. The 

 material for this was obtained as follows : — The powder obtained 

 from the hollows in the hexagons was boiled for a few minutes 

 in strong hydrochloric acid, and thoroughly washed in water : 



