234 CoLte—On the Geology of Slieve Gallion, in the County of Londonderry. 
red granite, much altered, occupies the head of the combes, with here and there a 
thin euritic zone where it abuts against the diabases of the plateau. It must be 
confessed that there seems nothing, in the arrangement of the rocks that have been 
spared to us by denudation, to explain the prevalence of eurite in the middle of 
the mountain-spur of Letteran. 
The specific gravity of the grey eurite of Letteran is 2°66; and its microscopic 
characters also connect it with the main mass of the granite. Some of the constituents 
are glomero-porphyritic groups, in which biotite, quartz, and altered felspar are 
associated precisely as in the granite of Ballybriest. The quartz crystals preserve 
their outlines fairly, as in typical ‘‘quartz-porphyries”; but they have undergone 
some corrosion from the groundmass, Orthoclase is the most prevalent porphyritic 
telspar, and the green biotite is also well developed and idiomorphic, except in the 
glomero-porphyritic groups. The groundmass is micropegmatitiec to micro- 
granular, without globular structure, and resembles in miniature the granite of 
Carndaisy Glen. 
Inclusions of green schistose material, doubtless derived from the diabase series, 
occur in the reddened form of this eurite 200 ft. higher up the slope; and a flinty 
rock, lying between the grey eurite and the boss of diorite, is possibly of mingled 
origin. Its specific gravity 1s as high as 2°78, and it contains a large amount of 
porphyritic plagioclase. ‘These crystals, however, are idiomorphic, and the ground- 
mass between them is-a micropegmatite rich im quartz. ‘here is rather more 
biotite, in streaks and irregular patches, than in the normal eurite; but nothing im 
the section is conclusive as to the mixed character of the mass. Possibly it 
represents the extreme of the granite series in approaching the quartz-aphanites, 
and may be compared with the brecciated rock under Slieve Gallion North. 
A more remarkable rock, illustrating the difficulties of the contact-zone, occurs 
at the junction, already referred to, west of the highest farm of Tintagh Hollow. 
It has the character of a grey eurite in the mass, with dull opaque white felspars ; 
but in section there is very little of the characteristic micropegmatitic groundmass. 
The coarser felspar, indeed, is in part associated with the quartz, as in the granite 
of Carndaisy Glen; but the groundmass round about such patches is supplied by 
a greenish fluidal material, which has no counterpart in the other eurites. In this 
ground, angular grains of quartz, and orthoclase and plagioclase felspars, are 
floated apart from one another; but it is often clear that adjacent graims belonged 
to one original crystal, and the structure thus resembles brecciation. ‘The quartz 
has been corroded by the green material, which enters into 1t in tongues; and this 
rock, with its two types of groundmass, proves to be a sigular product of interming- 
ling. As at Glasdrumman Port,* an attacked and remelted andesite has succeeded 
in attacking its invader, and the granite or micropegmatitic eurite has become 
* G. Cole, ‘‘ On derived Crystals, &c.,” Trans. Roy. Dubl. Soc., vol. v. (1894), p. 244. 
