1862.] 



HAUGHTON GRANITES OF IRELAND. 



413 



4. Black Mica. — Black mica forms in Donegal, as in the Mourne 

 Mountains, a constant and important constituent of the granite ; it 

 is always present, and becomes green when decomposition sets in. 



The following analyses show its chemical composition :-— 



Table YII. — BlacTc Mica of Donegal, 





No.l. 



No. 2. 



No. 3. 1 No. 4. 



Silica 



3616 

 19-40 

 26-31 

 0-58 

 4-29 

 0-48 

 900 

 0-62 

 0-40 

 2-40 



36-20 

 16-95 

 27-19 

 0-50 

 5-00 

 0-16 

 8-65 

 0-64 

 1-50 

 3-90 



44-40 

 21-52 

 10-72 

 2-70 

 6-14 

 0-74 

 6-18 

 3-96 

 1-28 

 1-20 



31-60 - 



19-68 



23-35 

 0-45 

 7-03 

 0-74 

 3-90 

 4-04 

 1-20 

 8-68 



Alumina 





Lime 





Soda 



Potash 



Iron fnrotoxide^ 



Manganese (protoxide) 





Totals 



99-64 



99-69 



98-84 i 100-67 1 







1 



No. 1. Glenveagh. — Occurs in coarse gneiss, containing also orthoclase and oli- 



goclase. 

 No. 2. Bally gihen. — Occurs in granite, in ^-inch plates, ^ inch in thickness. 

 No. 3. Garvary Wood. — Associated with oHgoclase, orthoclase, and molybdenite, 



in veins in gneiss. 

 No. 4. Castlecaldwell. — Associated with orthoclase and schorl in veins in gneiss. 



This mica is green, and is obviously the black mica much decomposed. 



Of these micas, ^o. 4 is evidently decomposing, and not to be 

 considered in forming an opinion on the average composition of the 

 black mica of Donegal. But the differences between No. 3 and 

 Nos. 1 and 2 are too numerous to permit us to take the average of 

 all. To enable us to form an opinion as to the probable composition 

 of this remarkable mineral, I here add four analyses, of which three 

 were made by myself : — 



Table VIII. — BlacTc Mica from other localities. 





No. 1. 



No. 2. 



No. 3. 



No. 4. 



SiHca 



35-55 

 17-08 

 23-70 

 3-55 

 1-95 

 0-61 

 307 

 0-35 

 9-45 

 4-30 



35-50 



20-80 

 21-40 



7-74 



39-70 

 12-25 

 23-55 

 0-96 

 100 

 4-48 

 7-25 

 0-47 

 7-30 

 1-00 



37-40 

 11-60 

 27-66 

 12-43 



1 0-26 



9-20 

 0-60 



AliiTm'Tia 



Iron (peroxide) 



Iron (protoxide) 



Manganese (protoxide) 



Ijime 



0-56 

 4-46 

 010 

 9-00 

 1-25 



Magnesia 



Soda 



Potash 



Loss by ignition 





1 Totals 



99-61 



100-81 



97-96 



99-15 





No. 1. Bally ellin, co. Car/ow.— (Haughton) Quart. Journ. Greol. Soc.vol.xii.p.175. 

 No. 2. Caw^ow.— (Haughton) Phil. Mag., April 1859, p. 259. 

 No. 3. Jonesed, Sv;eden. — (Haughton) unpublished. 

 No. 4. Pefersberg, Wermhmd. — (Soltmann). 



