Cote—On the Geology of Slieve Gallion, in the County of Londonderry. 219 
but the slide examined contains one fragment of devitrified perlitic glass. The 
association of these rudely stratified rocks with an obvious lava, and the character 
of their constituents, strongly suggest that they are volcanic ashes. The prepon- 
derance of felspar crystals may be due to the action of water on a more complex 
deposit, washing out the minute pumiceous fragments and leaving a crystalline 
voleanic sand. A similar phenomenon among the ashes of Cader Idris has already 
received this explanation.* 
A series of andesites is displayed upon Craigmore, the highest layer showing 
ashy characters; and the andesites in Brackagh-Slieve-Gallion, to north and west, 
become schistose here and there, and may contain a good deal of interbedded ash. 
A section of the rock west of Craigmore, at a height of 1250 feet, shows amygda- 
loidal scoriz embedded in a finer ground (PI. xtv., fig. 1). Some of the fragments 
contain large augite crystals, and show the nature of the original andesitic lava. 
The section thus confirms the first impression made by the rock in the field. South 
of Craigmore, across the road from Drumcormick to the White Water Pass, lies the 
picturesque mass of Windy Castle. In the hollow between this and the next 
ascent towards the pass, the andesites are again ‘associated with fragmental rocks, 
which reveal their structure on their weathered surfaces. These knobs of rock 
jut out upon the moorland that forms the east side of the Tintagh Hollow, and 
break the slope with a boldness rare upon Slieve Gallion, At 1330 feet we have 
a tuff, containing fragments of andesite 10 to 18 centimetres across; asin the 
previous case, the disrupted rock was a scoriaceous lava, now amygdaloidal, con- 
taining large porphyritic augites. Lower down the slope, the tuff becomes finer in 
grain, and in section the augite crystals are seen to lie separated in it, with 
scoriaceous fragments of various kinds of andesite, some rich in pyroxene, others 
rich in felspar. The lavas have become so far broken up as to set free the 
porphyritic augites ; and the rock reminds one, on a smaller scale, of the tuffs 
of Rhobell-y-Big,t an area with which Windy Castle may be compared in very 
many of its details. 
These rocks, especially in microscopic section, are not likely to be confused 
with the true breccias produced by earth-movement, which occur at other places 
in the andesitic series of Slieve Gallion. They form the nearest approach to sedi- 
mentary rocks that I can trace in the ‘“‘ Lower Silurian” areas; although, among 
the ‘‘pyroxenic rocks” on the east of the summit of Tintagh Mountain, dark 
bedded rocks occur, which may represent slightly ashy muds rather than muddy 
volcanic ashes. We must, indeed, cross the pass, and descend the White Water 
for some two and a half miles, before we find anything like a continuous stratified 
* Cole and Jennings, ‘‘ The Northern Slopes of Cader Idris,” Quart. Journ. Geol. Soc. London., vol. 
xlv. (1889), p. 427. 
+ G. Cole, ‘‘ The Rocks of the Voleano of Rhobell-fawr,” Geol. Mag., 18938, p. 342. 
