Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 221 
The andesites are usually compact lithoidal types, grey-green to deep grey in 
colour, and sometimes fissile and difficult to distinguish from sediments. Dark 
rocks, easily scratched with a knife, and resembling coarse argillaceous beds, occur 
thus, in intimate association with the diabases, on the east slope of Tintagh Hollow, 
on the eastern part of the crest of Tintagh Mountain, and on the southern edge of 
the plateau of Sleve Gallion South. Exposures in the higher moorland are too 
scattered to allow of the mapping of these rocks in detail; but I believe that they 
are merely modifications of the much altered lithoidal andesites, and their 
“bedding” is certainly in some cases due to subsequent earth-movement and 
brecciation. 
At Tirgan rock, the dark lava into which the granite has intruded is an andesite 
rich in small porphyritie erystals of augite. It has suffered from admixture with 
the quartzose intrusive mass. A type more rich in felspar is seen in Brackagh- 
Slieve-Gallion, crushed and brecciated, in the neighbourhood of the fault which 
earries down the Carboniferous sandstones to the north. Above Windy Castle, 
again, we have a glassy representative of the basaltic andesites—a delicately 
spherulitic rock, with plagioclase microlites imperfectly developed in it, their forked 
ends recalling the similar crystals so prevalent in variolites and modern tachylytes. 
The specific gravity of this rock, in its present somewhat devitrified condition, 
is 2°99. I have elsewhere described altered tachylytes from the coast of Co. Down 
with specific gravities ranging from 2°86 to 2°93. 
Another andesite, half a mile away, in a boss rising above the highest farm of 
Tintagh Hollow, shows in section a ground of green palagonite, and colourless 
microgranular material, with perlitic structure ; the small curved contraction-cracks 
are now darkened by the development along them of closely set granules of epidote* ; 
and dolomite, quartz, and white zeolites have arisen in the groundmass and in 
the fissures produced during alteration. The reference of the green groundmass 
to original tachylytic glass is borne out by the occurence of similar material in the 
hollows of the small porphyritic felspars. These hollows were certainly eaten out 
by the corrosive process of which we have evidence in almost every glassy lava. 
In its lower part, this rock has been affected by the granite, and quartz has 
filled up its steam-vesicles. Its microscopic characters are so different from those 
of ordinary perlitic glasses, that I have even compared the section with some of 
the oolitic iron-ores of North Wales, the grains of which are green in transmitted 
light. The features of the mass, however, in the field, and the obviously igneous 
origin of its crystalline constituents, make me put forward the above explanation 
as the true one. In its uppermost part the rock is coarsely vesicular, and the 
striking amygdales are formed of pale pink calcite. 
* Compare F. Rutley, ‘‘ On Perlitic Felsites from the Herefordshire Beacon, and on the origin of some 
Epidosites,” Quart. Journ. Geol. Soc. Lond., vol. xliv. (1888), p. 740. 
TRANS. ROY. DUB. SOC., N.S. VOL. VI., PART IX, 2L 
