On the Mineralogy of Dublin, and Wicklow. 



185 



separates it completely from Muscovite, of which it is considered 

 by Dana to be an altered variety. 



5. The Constituent Black Mica (Lepidomelane) of the granites 

 occurs in hexagonal plates, and is Uniaxial. Near BalJyellin, (Co. 

 Carlow) it is found associated with Margarodite in large plates ; 

 these plates are formed in about equal parts, of Lepidomelane 

 and Margarodite, which n't into each other at angles of 120° — 

 This fitting is purely mechanical and due t<5 the fact that the 

 angles of the Margarodite lozenges are GO and 120.° Lepidome- 

 lane is essentially an iron-potash mica, and is distinct from 

 Biotite, which is an iron magnesia mica. It is completely 

 decomposed by hydrochloric acid. It has the following chemical 

 composition : — 



Black Mica (Lepidomelane.) 



Silica, 



Alumina, 



Iron peroxide, 



Lime, 



Magnesia, 



Potash, 



Soda, 



Iron protoxide, . 



35 '55 per cent. 

 17.08 



Manganese protoxide, 

 Loss by ignition, • 



23-70 

 0-61 

 3-07 

 945 

 0-35 

 3-55 

 1-95 

 4-30 



99-61 



The granite axis of Leinster runs from Rockabill to Pouimounty, 

 N.N.E. to S.S.W., a distance of 90 miles. 



Eleven specimens taken at about equal intervals along this axis 



o-ave the following mean chemical composition : — 



Average Leinster Granite. 



Silica, . . . & 72*07 percent. 



Alumina, 



Iron peroxide, 



Lime, 



Magnesia, 



Potash, 



Soda, 



Loss by Ignition, 



100-08 



From this table, combined with the preceding tables, we obtain 



" 14-81 



5) 



\ 2-25 



)) 



1-63 



>J 



0-33 



JJ 



5-11 



JJ 



2-79 



5> 



1-09 



5? 



