ie 
VII. 
ON PITCHSTONE AND ANDESITE FROM TERTIARY DYKES 
IN DONEGAL. By PROFESSOR W. J. SOLLAS, LL.D., 
D.Sc., F.R.S. 
[Read DecemBeEr 21; Received for publication DrecemBEr 22, 1892; Published 
Marcu 26, 1893.] 
So far back as 1857 Dr. Haughton communicated to the Geo- 
logical Society of Dublin’ an account of some pitchstone dykes, 
which traverse the granite of Barnesmore Gap, Co. Donegal. 
Subsequently they were described by Mr. Kilroe of the Geological 
Survey,’ who speaks of them as numerous and as graduating from 
dark bluish grey glossy pitchstone to light pink or flesh-coloured 
compact felstone. ‘The specimens which furnished the material 
for Dr. Haughton’s investigations, which include a complete 
chemical analysis, are preserved in the Collection of the Geological 
Museum of Trinity College, and my attention being attracted by 
their remarkably fresh appearance, I had thin slices prepared 
from them, and these when examined under the microscope 
reveal a singularly close resemblance in structure and mineral 
composition to the celebrated pitchstones of Arran. Complete 
justification is thus afforded for the procedure of Sir Archibald 
Geikie, who has included these dykes of Donegal in his map of 
the Tertiary Volcanic areas of the British Isles.* 
Glossy black and vesicular in hand specimens, with occasional 
phenocrysts of sanidine and quartz, the pitchstone presents under 
the microscope (fig. 1) a colourless or brownish glassy base crowded 
with long slender needles of pyroxene, minute stellate crystallites 
1 Journal of the Geological Society of Dublin, vol. viz., p- 196. 
2 Memoir to accompany Sheet 24 of H.M. Geological Survey, 1888. 
8 Transactions of the Royal Society of Edinburgh, vol. xxxv., p. 184, 1889, 
