184 



Rev. Samuel Hatjghton, m.d., 



3. The Constituent Albite Feldspar has, hitherto, been found 

 in separate crystals, in one locality only (Dalkey) ; but it enters 

 largely into the composition of the granite rocks. 



Its chemical composition is as follows : — 

 Albite (Dalkey Quarry.) 



Silica, . . . 64-70 per cent. 



Alumina, . . . 21-80 „ 



Potash, ... 2-84 „ 



Soda, . . 9-78 



Fluorspar* . . . 0'80 



90-92 



70 



o 



4 



72 



18 



70 







67 



11 



76 



15 



4. The Constituent White Mica (Margarodite) of the granites, 

 ofien occurs in liat rhombic prisms, or in hexagonal plates, formed 

 from the former by the replacement of the acute angles ; the 

 angles of the lozenges are 120° and G0° ; and the crystals are 

 Biaxial, the plane of the optic axes tracing the major diameter 

 of the lozenge. 



The following measurements of the angle between the optic 

 axes have been recorded : — 



1. Three Kock Mountain, 



2. Glendalough, 



3. Mount Leinster, 



4. Lough Dan, 



5. Glenmalure, 



6. Poulmounty, 



The average chemical composition of the white mica is as 

 follows : — 



Margarodite (Mean of four specimens.) 



Silica, 



Alumina, 



Iron peroxide, . 



Lime, 



Magnesia, 



Potash, 



Soda, i • . 



Loss by ignition, « . 



.-.■-. • 99-78 

 The amount of water of crystallisation present in this mineral 



* The Albite was found in small crystals, lining cavities in the granite, and encrusting 

 crystals of Orthoclase ; and it was associated with similar small crystals of accidental 

 purple fluorspar, from which it was separated with difficulty. 



32-13 



CL Ut 



4-57 





0-78 



7} 



0-76 



>) 



10-67 



77 



, . 0-95 





. • 5-34 



)f 



