CoLe—On the Geology of Slieve Gallion, in the County of Londonderry. 239 
developed red-brown eurite, with porphyritic quartz, comes in as the edge of the 
important mass of Crockandun. 
One band of the seemingly intrusive and highly siliceous series in the south of 
Beaghbeg is grey-green and almost translucent, and closely resembles an inter- 
bedded chert. This is probably the ‘‘hornstone” of Portlock.* 1 submitted a 
section from it, with those of cherts from Slieve Gallion, to Dr. G. J. Hinde, 
F.R.S., who has very kindly examined it. He writes that some of the clear 
patches in it ‘‘ correspond in size and outline with radiolarian casts, but most of 
them are without any likeness whatever, and I do not think any of them are of 
organic derivation.” ‘This observation corresponds with the evidence in the field, 
and I feel confidence in referring this rock to the highly siliceous intrusive series. 
It was probably at one time an obsidian; and the ovoid bodies in it, more 
transparent than the ground in microscopic section, are most likely chalcedonic 
infillings of minute vesicles. Their arrangement along certain planes supports this 
view; and veins infilled by similar chalcedony traverse the whole mass. 
I fear, then, that we lose even this rock as a possible marine sediment; and the 
dark schistose rocks of the higher part of the townland are even more certainly 
voleanic. The very summit of Beaghbeg is formed of a rudely fissile dark grey 
rock, resembling a poor slate passing towards the phyllite stage. A section of 
this proves it to have been once a trachyte, with a groundmass like that of the 
familiar rocks of Ischia; and porphyritic crystals of orthoclase and of a triclinic 
felspar are embedded in it. The minute green crystals of the groundmass, which 
was once fairly glassy, are probably soda-augite, porphyritic representatives of 
which are absent. The rock has now a specific gravity of 2°69, lower than that 
of the andesites of Slieve Gallion, and is, in my experience, unique in the whole 
area examined. 
Below this trachyte on the crest, and also beside the stream bordering on 
Meenascallagh, the remarkable “‘ breccias” of Beaghbeg are exposed. These are 
well described by Mr. Nolant in their aspect in the field, and by Sir A. Geikie,t 
who states that they pass into ‘“ green schists,” like those of Argyllshire and the 
central Scottish highlands. Sir A. Geikie regards these beds, with those of 
Creggan Lough, as volcanic agglomerates, containing both acid and basic rocks ; 
and he points out the occurrence of vesicular structure in the blocks, when seen in 
microscopic section. Messrs. M‘Henry and Watts§ also refer to these rocks as 
probably volcanic products. 
* Op. cit., p. 533. 
+ ‘‘Metam. Rocks of Tyrone,” Geol. Mag., 1879, p. 157; compare those near Creggan Lough, Mem. 
sheet 26, p. 14. 
{ Anniversary Address, Quart. Journ. Geol. Soc. Lond., vol. xlvil. (1891), Proceedings, p. 77. 
§ Op. cit., p. 59. 
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