the Mica of the Dublin, Wicklow, and Carloio Granites. 275 



of the Three Rock Mountain, and this deviation may possibly be 

 due to the presence of grains of black mica, which also occurs in 

 the mass of the granite, and of which I was unable to obtain a 

 sufficient quantity for chemical or optical examination. The 

 quantity of protoxides in the Three Rock mica is somewhat in 

 excess of that required by the formula. 



The angles between the optic axes of these micas were care- 

 fully determined, and found to be as follows : — 



Angles between Optic Axes of Micas. 



I have added to the determination of angles of the micas 

 analysed, the optic angles of two other specimens of gray, 

 transparent mica from Lough Dan and Glenmalure, county 

 Wicklow. 



The four micas which were free from any intermixture of 

 black mica have a high angie, while the angle between the optic 

 axes of the mica from the Three Rock Mountain, which con- 

 tained flakes of black mica, is nearly 20° less than that of the 

 pure transparent gray micas. 



The uniformity of the preceding analyses is sufficient to show 

 that margarodite is entitled to be considered as a distinct 3pecies 

 of hydrated mica, and that it is not merely an altered form of 

 muscovite, an opinion advocated by M. Dana in the last edition 

 of his ' System of Mineralogy.' 



Crystallographically considered, the micas just described are 

 triinetric, and occur in flat, right rhombic prisms, with angles of 

 6° and 120° j or in hexagonal tabular prisms, formed by the 

 replacement of the acute angles of the rhombic prisms, all the 

 angles of the hexagon being 120°. 



The plane of the optic axes is perpendicular to the flat plane 

 of cleavage, and bisects the acute angles of the rhomb, or inter- 

 sects perpendicularly two of the sides of the hexagon. I have 

 not found among the micas of the granite of the south-east of 

 Inland, any instance of the optic axes lying in the shorter 

 diagonal of the rhomb. 



T2 



