Oiv the Mineralogy of Dublin und Wicklow. 185 
separates it completely from Muscovite, of which it is considered 
by Dana to be an altered variety. 
5. The Constituent Black Mica (Lepidomelanc) of the granites 
occurs in hexagonal plates, and is Uniaxial. Near Ballyellin, (Co. 
Carlow) it is found associated with Margarodite in large plates ; 
these plates are formed in about equal parts, of Lepidomelane 
and Margarodite, which fit into each other at angles of 120°— 
This fitting is purely mechanical and due to the fact that the 
angles of the Margarodite lozenges are 60° 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 folloy wing chemical 
composition :—- 
Black Mica (Lepidomelane.) 

Silica, : ; ; 30°09 per cent. 
Alumina, ; : ‘ 17.08 3 
Iron peroxide, . F ; 23°70 ie 
Lime, : ° . 0-61 Fe 
Magnesia, : : : 3°07 ', 
Potash, ; . ‘ 9°45 +5 
Soda, . : 3 0°35 % 
Tron protoxide, . , , 3D rf 
Manganese protoxide, 1:95 5 
Loss by ignition, . 4°30 BS 
99°61 
The granite axis of Leinster runs from Rockabill to Poulmounty, 
N.N.E. ie S.S.W., a distance of 90 miles. 
Eleven specimens taken at about equal inter vals along this axis 
gave the following mean chemical composition -— 
Average Lewmster Granite. 
Silica, ; : - _*42°07 per cént. 
Alumina, ° . go EA Sie 
Tron peroxide, ‘ : . yo tsy eke 
Lime, . . ‘ bOSmiics, 
Magnesia, . F ‘ A) Og ares 
Potash, ; . : cid 8 ac ah 
Soda, : . ‘ tO malts 
Loss by Ignition, . . : Oot rin 

100-08 
From this table, combined with the preceding tables, we obtain 
