220 Cote —On the Geology of Shieve Gallon, in the County of Londonderry. 
series. Even then, among the schistose layers by the stream, on the southern 
side of Straw Mountain, we may possibly mistake foliation-planes for original 
bedding. A section prepared from one of these greenish rocks shows that con- 
siderable alteration has set in; secondary quartz fills all the cracks and cavities, 
and the main mass is largely chloritised. The structural planes were probably 
impressed upon the rock by shearing ; and in its original condition it may easily 
have been an andesitic ash. he dip of the structure-planes is north; and this is 
repeated in the bluffs on the White Water, a mile above the bridge, on the ascent 
towards Slieve Gallion. All these greyish, greenish, and pinkish rocks strongly 
suggest volcanic ashes, and I believe that true bedding occurs among them. At 
the same time, a compact diabase, associated with them on the upper White Water, 
shows in section such evidence of crushing, with consequent production of struc- 
tural planes, that I am willing to suspend judgment until the series can be traced 
farther north. The fine-grained schists of the Sperrin Mountains, so far as I have 
seen them in the field, differ widely from the White Water series; and their mica- 
ceous foliation-planes place them as distinctly metamorphic. 
I attach considerable importance to the tuffs of the Windy Castle area, especially 
in connection with the vesicular andesites that are associated with them. It seems 
difficult to believe that the latter, so carefully noted by Mr. Egan,* should have 
raised no suspicion of the yoleanic origin of the series. 
The andesites themselves, and their more crystalline representatives, the apha- 
nites, are first met with at a height of 500 feet in the east of Brackagh-Slieve- 
Gallion, at 575 feet in the adjacent Tirgan Rock, and at 1075 feet on the south 
side of Windy Castle. In the last two cases, they are in contact with the under- 
lying granite, which rises round Sheve Gallion South to 1600 feet. Hence the 
andesitic series may be either a comparatively thin one, bent down on the flanks 
of the mountain and forming a mere arch above the granite core; or it may be one 
of variable thickness, eaten away to a greater or less extent by the invading granite 
on its lower side. The frequency with which signs of contact-alteration are seen In 
the field, as we ascend for 1000 feet over continuous andesitic rocks, indicates 
that we are nowhere far distant from the granite; and I doubt if the series, as it 
remains to us, is at any point thicker than 200 feet (Pl. xm). 
The rocks in contact with the granite are by no means of uniform character ; 
but they have generally become flinty, and some of the ash-beds at Drummuck and 
Craigmore are almost porcellanous, and can no longer be scratched with a knife. 
In this condition they weather almost as white as the altered rhyolitic ashes of 
Cader Idris. 
* Mem. sheet 27, p. 12. 
+ Mr. Nolan also notices amygdaloids among the schistose rocks of Sleve Gallion (*‘ Rocks of Tyrone,” 
Geol. Mag., 1879, p. 156). See also Portlock, ‘Geol. of Londonderry,” pp. 545 and 542. 
