1862.] HAUGHTON GRANITES OF IRELAND. 



Table V. — Orthoclase of the Donegal Granite. 



411 





1. 



2. 



3. 



Mean. 



SiUca 



Alumina .. . 



63-20 

 19-72 

 0-28 

 2-59 

 0-09 

 0-06 

 16-30 



62-80 

 16-84 

 0-96 

 4-95 

 0-11 

 0-46 

 14-91 



63-60 

 19-32 

 0-80 

 0-72 

 0-14 

 1-84 

 13-55 



63-20 

 18-64 

 0-68 

 2-75 

 0-11 

 0-78 

 14-92 



Iron (peroxide) 



Lime 



Magnesia 



Soda 



Potash 





Total 



102-24 



101-03 



99-97 



101-08 , 



No. 1. Glenveagh. — ^White, opaque, milky, forming crystals in the granite. 



No. 2. Near Lough Mourne, above Barnesmore Gap. — Found in great bunches, 

 isolated, in the middle of a very close-grained mica-schist, or gneiss of very 

 fine grain. The felspar is bright-red, and associated with milky quartz, 

 containing specular micaceous iron -oxide. The diameters of some of 

 the bunches are 5 ft. They are probably the terminations of veins 2 ft. 

 wide, ending in carbonas in the gneiss, and have all the appearance of 

 having been filled by aqueous action at a high temperature. 



No. 3. Castlecaldwell. — Found associated with white mica, quartz, black mica, 

 and occasionally schorl and iron-pyrites, in veins penetrating the fine- 

 grained gneiss of the district. The felspar of these veins is worked for 

 the manufacture of china, and burns white, although pink and red in 

 the vein. 



Taking the atoms of the mean composition of the orthoclase, I 

 find— 



SiHca 1373 .... 35 



Alumina ........ 358 1 



Iron (peroxide) . . 8 J 



Lime 98 " 



Magnesia 5 . . . ^ ., ., 



Soda 25 ^ • • • • ^^^ • • • • ^^ 



Potash 317 



The protoxide bases are here somewhat too large for orthoclase ; 

 and it is probable that this is caused by the lime of No. 2, which is 

 unusually large in amount. There can be no doubt, however, on 

 crystaUographic grounds, that the felspar is true orthoclase. 



For the purpose of comparison, I here give the mean of seven 

 analyses of different orthoclases of the Leinster granite, and their 

 atomic composition : — 



Leinster Orthoclase {mean of seven analyses). 



Atoms. 



366 



Per-centage. 



Silica 64-59 



Alumina 18-31 



lime 0-25 



Magnesia .... 0-58 



Soda 2-75 



Potash 12-23 



Loss by ignition 0-58 



29 



89 I 



260 J 



1435 

 352 



387 



36 

 9 



10 



99-29 



The chief difference between the orthoclase of Donegal and that 



