412 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuHG 18, 



of Leinster consists in the fact, that lime seems in the Donegal 

 mineral to take the place of the soda in the Leinster felspar, and that 

 it is somewat more basic. 



3. Oligoclase. — The oligoclase of Donegal is of a honey- waxy- 

 greenish grey, and is easily distinguished from the orthoclase which 

 accompanies it by its colour and by the fine striated lines that mark 

 certain of its surfaces of crystallization, and prove it to be an an- 

 orthic felspar. The following analyses give its composition : — 



Table YI. — Donegal Oligoclase. 



Silica 



No.l. 



No. 2. 



Mean. 



Atoms. 





60-56 

 24-40 

 0-40 

 5-96 

 0-04 

 6-46 

 1-76 



59-28 

 22-96 

 1-94 

 4-65 

 0-21 

 6-48 

 2-38 

 0-10 

 0-32 



59-92 

 23-68 

 1-17 

 5-30 

 0-13 

 6-47 

 2-07 

 0-05 

 0-16 



1303 



It} 469 

 189 > 



6 

 209 I ,., 

 44 r ^^4 



1 6 

 J / 



3 

 1 



1 



Alumina . . 



Iron (peroxide) 



Lime 



Magnesia 



Soda 



Potash 



Iron (protoxide) 



Manganese (protoxide) 



Totals 



99-58 



98-32 i 98-95 









No. 1. Garvary Wood, near Castlecaldwell, Co. Fermanagh. — Pearl-grey, trans- 

 lucent ; in reins in gneiss ; associated with black mica, some orthoclase 

 (pink), copper-pyrites, and molybdenite. 



No. 2. Precise locality unknown. — The specimen from which it was taken belongs 

 to that variety of granitic syenite into which the granite of Donegal 

 sometimes passes, as at the Black Gap, Pettigo, and at Kilraine, near 

 Ardara. 



The formula to which the preceding analyses lead is the well- 

 known formula of oligoclase — 



R(),Si03 + A1^03,2Si03. 



For the purpose of comparing the oligoclase of the granite of Donegal 

 with that of Sweden, I made a careful analysis of the oligoclase of 

 Ytterby in Sweden, which was kindly placed at my disposal by Mr. 

 J. B. Jukes, Local-Director of the Geological Survey of Ireland. 



The following is the mean of two closely agreeing analyses : — 



Oligoclase^ from Ytterby, Sweden. 



Silica 



Per-centage. 

 . . . 63-66 



A 



toms. 

 . 1415 



Alumina . . . 



. . . 23-45 







. 451 



Lime 



. . . 3-53 



. . . 126 ^ 





Magnesia. . . 

 Soda 



0-05 



. . . 7-91 



2 

 ... 255 



. 417 



Potash 



. . . 1-59 



34, 





100-19 



* Throughout the entire mass of the large crystals of oligoclase examined, minute 

 specks of quartz were occasionally visible ; a circumstance which seems to me 

 irreconcileable with the supposition of the formation of this oHgoclase by fusion, 

 in the dry way. 



