[ 116 ] 



XVIlT. Notes on Mineralogy. — No. IV. On the Pitchstone Por- 

 phyry of Lough Eske, Co. Donegal. By the Rev. Samuel 

 Hatjghton, M.A., Professor of Geology in the University of 

 Dublin^-'. 



DURING a visit to the Co. Donegal in the summer of 1856, 

 I observed a remarkable series of dykes of felspatbic trap 

 and porphyry intersecting the granite of Barnesmore Gap, ex- 

 hibiting occasionally a tendency to pass into a description of 

 glossy pitchstone. Subsequently I was favoured by James Wood, 

 Esq., of Castlegrove, with some specimens from the mountains 

 beyond Lough Eske, in the same locality, which are genuine 

 pitchstone, passing into amygdaloidal, or rather oolitic porphyry, 

 the cavities being filled with a white mineral which I consider to 

 be stilbite. As the locality is a new one for pitchstone, and the 

 mineralogical composition of the i-ock unusual, I thought it 

 might not be uninteresting to the readers of the Philosophical 

 Magazine to place on record its analysis, and the result of my 

 discussion of that analysis. 



Analysis of Pitchstone from Lough Eske, Co. Donegal. 

 Per-centage. Atoms. 



Silica 6401 1-4.23 



Alumina 10-40 0-2041^0^0 



Peroxide of iron . . . 9-36 0-104 J ^ "^^^ 



Lime 424 0-1511 



Magnesia none ... I „ ^^n 



Potash 3-63 0-077 f ^'^"^^ 



Soda 2-91 OO94J 



Loss by ignition . . . 5-13 0-570 



99-71 

 Assuming this rock to be composed of quartz, felspar and 

 stilbite, and writing Q, F, S for the number of atoms of each 

 mineral respectively, vvc find, since 

 Quartz = SiO^, 



Felspar = RO, SiQa + R^O^, 3SiO^ 

 Stilbite = RO, SiO^ + R'^ O", SSiO^ + 6110, 

 Q^.4F + 4S = 1-423, 

 F + S = 0-315, 

 6S = 0-570. 

 From these equations, we obtain readily, 

 Q = 0-163, 

 F = 0-220, 

 S = 0-095. 

 * Communicated bv the Author. 



