226 Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 
A great part of the intermingling observed in the field is associated with the 
presence of visible veins of granite or eurite, and will be dealt with in the next 
section of this paper. The aphanite and diorite of the south side of the Letteran 
Hollow present, however, features of greater delicacy. 
These rocks form a conspicuous boss on the shoulder descending south-east 
from Slieve Gallion South, extending upwards for about 100 feet from a spot 1225 
feet above the sea. They are completely surrounded by the granite or its euritic 
representatives, which reach the level of 1600 feet upon the slope above. The 
exposure measures only some 500 feet by 400 feet when set down upon the map, 
a small thing on the great side of the mountain (PI. xm1.). 
Hornblende-diorite is seen at the foot of the little cliff, just against the bend 
of the cart-track to the higher bog. The summit of the boss is, however, aphan- 
itic; and the rock is here clearly related to the normal diabase series of Slieve 
Gallion. 
There are pink felspars in the diorite, as well as in the surrounding granite ; 
but microscopic examination shows that these must not be attributed to the intro- 
duction of granitic material. Sections prepared from different parts of the boss 
give, however, the following evidences of intermingling. First, quartz occurs in 
very varying quantity throughout the mass; it has moulded itself against the 
pre-existing altered felspars. Secondly, the quartz is in places associated with 
another felspar, asa micropegmatitic intergrowth, which forms a network of 
delicate veins. Thirdly, in the neighbourhood of the quartz, the altered prismatic 
felspar is commonly ‘‘ restored” by an outer clearer zone, in which the repeated 
twinning of plagioclase is sometimes seen; its cracks have similarly been infilled 
with fresh felspathic material. 
Fourthly, the ferromagnesian material is a green or brown-green hornblende, 
which is somewhat irregularly developed in contact with the prismatic felspars, 
and which retains pale cores here and there strongly suggestive of original 
pyroxene. On the other hand, in contact with the quartz, the hornblende is 
strikingly idiomorphic, and the best crystals are found almost surrounded by 
quartz (PI. x1v., fig. 4). 
The conclusion is that the original rock was a crystalline representative of the 
andesitic series which lies on the higher levels of the mountain, and consisted of 
pyroxene and plagioclastic felspar. The latter is too much decomposed for 
specific determination; the former has passed into green hornblende. The 
magma of the invading granite became intimately injected through the mass, and 
some of its chemical constituents were withdrawn to form fresh zones and 
infillings to the old felspars. The secondary hornblende of the diorite received 
new life; it was doubtless strengthened by similar material from the granite, and 
crystallised out boldly towards the intrusive veins. Finally, quartz, and occasion- 
ally micropegmatitic material, separated out in the remaining spaces, and bound 
