414 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 18, 



Kammelsberg (^Handbuch der Mineralchemie,' 1860, p. 668) gives, 

 in his list of magnesia-micas, three only that can be compared with 

 the eight analyses just recorded — viz. Nos. 23, 24, and 25, although 

 he does not hesitate to include in his list micas whose composition 

 with respect to magnesia ranges from 3 to 30 per cent. Physically, 

 it is evident that the eight micas, whose composition is given in the 

 preceding Tables, are identical, notwithstanding the variation in their 

 chemical composition. 



5. White Mica. — This mineral, although not a constituent mineral 

 of the granite of Donegal, occurs frequently in veins, and is always 

 associated with orthoclase, sometimes with schorl and beryl. It is 

 biaxial, and resembles the margarodite of Leinster already described 

 in Part I. of these Researches. 



The following analyses show its composition : — 



Table IX. — White Mica of Donegal. 





No.L 



No. 2. 



Mean. 



Silica 



44-80 

 29-76 

 8-80 

 0-45 

 0-71 

 0-32 

 12-44 



45-24 

 35-64 

 2-24 

 0-51 

 0-71 

 0-54 

 10-44 

 0-70 

 0-24 

 400 



4502 



1 38-22 



0-48 



0-71 



0-43 



11-44 



1 0-71 



3-00 



Alumina 



Iron (peroxide) 



Lime 



Magnesia 



Soda 



Potash 



Iron (protoxide) 



Manganese (protoxide) 



Loss by ignition 



0-48 

 2-00 





Totals 



99-76 100-26 



100-01 









No. 1. CastlecaldwelL — Found in veins of quartz and pink orthoclase, con- 

 taining schorl and decomposing plates of black mica. Biaxial (72° 20'). 

 The accompanying woodcut (fig. 1) shows the position of the plane of 

 optical axes. Angle of plate = 1 25°. 



No. 2. Near Ballygihen, in Dooish Mountain. — In veins in the granite, not 

 associated with black mica ; in plates f inch wide, ^ inch thick. Bi- 

 axial (62° 10' to 65° 10'). Angle of plate = 120°. 



Fig. 1. — White Mica from 

 Castlecalclwell, Donegal. 



Fig. 2. — White Mica from Breezy 

 Mountain, near BeleeTc. 



In a specimen of speckled white mica from Breezy Mountain, near 

 Beleek, I found the optical axes to form an angle of 63° 6' ; and 

 also observed their plane to coincide with the shorter diagonal of 

 the primary rhomb, whose angles were 120° and 60°. This is shown 



