Prof. G. A. J. Cole— The Volcano of Tardree. 305 



But the northern side of that volcano shows a far more striking 

 assemblage of rhyolites, among them being the exquisitely perlitic 

 glass recently dealt with by Mr. W. W. Watts.^ This is the 

 " pitchstone porphyry " and " pearlstone porphyry " of Sandy Braes, 

 noted by Berger,'^ Portlock,^ and other older authors. 



It is surprising how the spread of petrography on the Continent 

 has sent us to Hungary or Lipari for our specimens of rhyolite, 

 while the rich variety of material on Sandy Braes has beeu scai'cely 

 known to the collector. 



Von Lasaulx was acquainted with the glassy type of the rhyolites 

 of County Antrim merely from specimens that were shown to him in 

 Dublin; and few of the eminent geologists who have visited the 

 quarries of Tardree appear to have climbed thence to the plateau of 

 Sandy Braes. This upland is traversed by a bold mountain-road, 

 commanding superb views, and running from Doagh to Connor ; 

 and a steep lane rises up to it just east of the inn on the north side 

 of Tardree Mountain. The name Sandy Braes does not occur on 

 the ]-inch Ordnance Map, but is easily found upon the 6-inch 

 sheet ; it has a real meaning to the local farmers, who quarry the 

 decomposing rhyolites for sand. 



On the steep cross-road referred to, there is a little quarry, which 

 is visible from the Tardree inn ; the rock is a pink and well-banded 

 rhyolite, its planes of flow being distinctly visible at a distance. 

 Above this, the surface of the moorland is strewn with boulders, 

 large and small, consisting of grey, black, and greenish obsidian, 

 sometimes perlitic, rarely spherulitic, and always containing 

 porphyritic quartz and felspars. This rock breaks up easily under 

 the hammer, and its numerous joint-planes and its partial decom- 

 position cause it to weather down into a yellowish sand. This is 

 the material excavated for use on the paths of local demesnes, and it 

 can be seen in the casual little diggings that are made from time to 

 time. These diggings do not go farther down than the first hard 

 rock, so that their walls show merely lumps of glassy rhyolite 

 embedded in soft sand ; but their floors consist of surfaces of 

 decomposing glass, or of compacter and firmer rhyolite, like that 

 of the quarry on the ascent. From the crest of this ascent, across 

 the Doagh road, and away to Sandy Braes, the rock below the 

 surface of the grass or heather is almost pure obsidian. It is 

 probable that the flat dome-like summit forming the southern 

 extension of the braes originated mainly in the surface of a 

 single lava-flow, which was formerly connected with the central 

 mass at Tardree Mountain. 



Compact rhyolites occur as we push across to Sandy Braes, and 

 serve to show how the crust of the obsidian, which is now mostly 

 broken up into boulders, is in reality only two or three feet in 

 thickness. At the east end of the braes there is a red compact 



1 " The Occurrence of Perlitic Cracks in Quartz," Q.J.G.S. vol. 1 (1894), p. 368. 

 ^ " On the Geological Features of the North-eastern Counties of Ireland," 

 Trans. Geol. Soc, ser. i, vol. iii (1816), p. 190. 



^ " Report on the Geology of Londonderry, etc.," 1843, p. 212. 



DECADE IV. VOL. II. — NO. VII. 20 



