Cote—On the Geology of Slieve Gallion, in the County of Londonderry. 223 
nature of the groundmass. A third example may be taken from the crest of the 
south-west wall of the Letteran Hollow, in Letteran itself; its specific gravity is 
only 2°76. The granite lies close below it ; but no intermingling of material seems 
to have occurred. Moreover, the phonolitic aspect of the rock-mass is not confined 
to these hornblende-andesites. The flinty augite-andesite of Tirgan Rock, and a 
pinkish-grey mass on the bluffs along the upper White Water, were equally set 
down in the field as possible phonolites. In both these cases, the granite has sent 
off veins into the rock ; and the compact character of the whole “ pseudo-phono- 
litic”” series seems due to contact-metamorphism acting on a groundmass of similar 
chemical composition, independently of the mineral constitution of the lava as a 
whole. 
When passed in review, the hornblende-andesites of Slieve Gallion seem to 
have originally belonged to a type with trachytic structure, while the augite- 
andesites, which occur more commonly,* approach the basalts. But some among 
the latter are grey and very rich in small porphyritic felspars, being hemicrystal- 
line representatives of the rocks styled ‘‘ leucophyres” by Giimbel.t One of 
these paler andesites, from the head of the Letteran Hollow, gives, even under 
the microscope, no clue as to its original ferromagnesian constituent, irregular 
chloritic patches alone remaining. It has been thoroughly permeated by quartz, 
in insidious veins and streaks, and les close above the interesting green chert- 
band which accompanies the diabases in that hollow. Veins of hematite also 
traverse the rock; but these various changes have not disguised the original highly 
felspathic nature of the rock. A more satisfactory example of these ‘ andesites 
rich in felspar” occurs among the dark compact rocks of the eastern part of the 
crest of Tintagh Mountain. It is traversed by numerous eurite veins; but enough 
of its structure remains clear. One or two porphyritic crystals of augite appear, 
while plagioclase, in short stout prisms, is simply abundant. The groundmass 
was originally minutely vesicular; and some remelting seems to have gone on in 
contact with the invading veins. 
The compact rock which has been already referred to, from the bluffs on the 
White Water, two-thirds of a mile from the pass, is now much brecciated, but was 
originally one of the highly felspathic andesites. Its ferromagnesian constituent 
has been lost in epidote and chlorite. 
Enough has now been said to show that, from end to end of Slieve Gallion, 
the whole series of normal andesites is fairly represented. 
There remain, however, certain rocks more rich in silica, which approach the 
rhyolites ; but they occur very sparsely, and I believe that their quartzose character 
* Portlock regarded hornblende as the common constituent throughout Slieve Gallion, having correctly 
observed it at Windy Castle and elsewhere (‘‘ Geol. of Londonderry,” pp. 210 and 538). 
+ See discussion by Rosenbusch, ‘‘ Mikr, Phys. der massigen Gesteine ” (1887), p. 200. 
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