248 CoLte—On the Geology of Slieve Gallion, in the County of Londonderry. 
PLATE XIII. 
View of Slieve Gallion from the south-east, coloured to show the distribution of the various igneous 
and sedimentary series. 
PLATE XIV. 
Fic. 1. Microscopic section of augite-andesite tuff, from flinty and bedded mass above Craigmore, Slieve 
Gallion. The left hand upper portion shows a lump of andesite with porphyritic felspar; the 
right-hand lower mass is amygdaloidal. The fine ashy ground, with signs of compression and 
flow, is seen in the interspace. x 10. 
Fic. 2. Microscopic section of rhyolite, from dyke in Oritor Quarry. A porphyritic felspar, with small 
corroded quartz-grains above it, occurs in the left-hand upper portion. The flow-structure is 
more distinctly seen in the original. x 8. 
. Microscopic section of hornblende-diorite (altered gabbro) penetrated by veins of micropegmatite, 
from boulder on S.W. slope of Glenarudda Mountain, Sheve Gallion. The normal structure 
of the diorite, with its ‘‘saussuritic’’ felspar, is seen below ; while the invasion of the micro- 
pegmatitic magma, in the upper part of the section, has induced a free and idiomorphic 
crystallisation of the hornblende of the rock attacked by it. x 8. 
Fie. 
oo 
Fic. 4. Microscopic section of hornblende-diorite penetrated by micropegmatite, from Letteran, Slieve 
Gallion. In contact with the quartz of the micropegmatite, the hornblende has assumed its 
characteristic outlines, although it occurs in a fibrous uralitic condition in the other parts of 
the rock. The original plagioclastic felspars have also renewed their growth in many places, 
as may be seen with a higher power. The rock is an example of a quartz-hornblende- 
diorite produced by admixture of an altered pyroxene-diorite (or gabbro) and a granite magma. 
xml 
Fic. 5. View of dyke of pink rhyolite traversing basic rocks at Oritor Quarry (see fig. 2 above). The 
dyke runs obliquely upwards from the right. 
Fic. 6. View of part of bosses of altered gabbro, traversed and highly penetrated by veins of micro- 
pegmatitic eurite and aplitic granite. Opposite the farm of Keragh, Beaghbeg, Co. Tyrone. 
