222 Cote—0On the Geology of Slieve Gallion, in the County of Londonderry. 
Vesicular andesites occur on Drummuck, Windy Castle, and Craigmore, having 
been rendered amygdaloidal, and then restored by weathering. Their association 
with the tuffs of this area, which are characteristically full of scoriaceous fragments, 
makes it safe to regard them as true lava-flows. On the north-western rocks of 
Windy Castle, a vesicular hornblende-andesite occurs, in place of the common 
pyroxenic varieties. 
Vesicular structure is, indeed, fairly frequent among the andesites of Slieve 
Gallion. A striking example occurs on the very summit of Sheve Gallion South, 
as a layer dipping south among compacter masses. ‘This rock is in part actually 
scoriaceous. The summit of Glenarudda Mountain consists of a delicate amygda- 
loid, the small vesicles being filled by quartz, epidote, and pyrite. A vesicular 
patch, again, is exposed among the more compact diabases in the bog above the 
hollow of Letteran. 
On the higher levels of Slieve Gallion, west of the White Water Pass, horn- 
blende-andesite is fairly common, forming the layer in contact with the granite. 
The hornblende is clearly an original constituent, and has not arisen as a contact- 
product. One example of this group of andesites occurs, with signs of brecciation, 
on the east of the crest of Tintagh Mountain; but the most typical exposures lie 
round and on Slieve Gallion South. The rock is non-scoriaceous and compact, and 
its groundmass resembles, in a remarkable degree, that of the nepheline-phonolites. 
In grain it is at the best only slightly coarser than the well-known Bohemian 
phonolites, and the dull grey-green colour forms an additional point of similarity. 
On weathering, the groundmass often becomes a pale pinkish white. Even under 
the microscope, the extremely altered character of the felspathic constituent, and 
the short habit of its crystals, suggest the presence of porphyritic nepheline ; and 
it was with reluctance that I abandoned this idea, upon finding traces of repeated 
twinning in several of these decomposing crystals. The plagioclase, in fact, has 
been largely replaced by calcite, and the rock effervesces briskly, even when treated 
with cold acid. The hornblende, on the other hand, preserves its character 
fairly, though its green colour is likely to be due to incipient changes towards 
chlorite. 
The compact hornblende-andesite of the summit of Slieve Gallion South, 
where there is a somewhat considerable rock-exposure, has a specific gravity of 
2°82. In section, the glassy ground is represented by a colourless microgranular 
modification, in which the microlites and crystallites of the original glass have 
been preserved in all their delicacy. 
In Mobuy, on the slope above Lough Fea, 530 feet lower than the last-named 
exposure, a similar but somewhat coarser andesite occurs, again in contact with the 
granite. Its specific gravity is 2°85, which is partly accounted for by the presence 
of epidote. The cumulites and other crystallitic groupings again show the former 
