368 MB. W. W. WATTS ON THE OCCURRENCE [Aug. 1 894, 



II. Macroscopical Characters. 



The ordinary type of Tardree rhyolite is well known ; it is a light 

 grey, or pinkish-grey, trachytic rock showing fair-sized crystals of 

 sanidine, smaller crystals of plagioclase, and some quartz, any ferro- 

 magnesian mineral being decidedly rare. 



The variety from Sandy Braes, which I have more particularly to 

 describe, would be perhaps more appropriately termed a porphyritic 

 pitchstone (or obsidian), composed of a pitch-black glass, with a 

 lustre generally brightly vitreous, but occasionally more resinous. 

 The perlitic structure is clearly seen with a lens, and as the rock 

 fractures the projecting perlites stand out on its surface. Where the 

 perlites are broken across the glass is seen to become much paler, 

 and occasionally quite white towards their interior. Whilst the 

 felspars break along cleavage-planes, it is seen that the quartz, 

 instead of showing its customary conchoidal fracture, stands up in 

 rounded grains, and is traversed by cracks roughly concentric with 

 this outer surface. In some specimens there are a good many patches 

 of a whitish substance, which may be devitrified glass. In specimen 

 1 927 large irregular cracks with wavy surfaces are seen to enclose 

 the perlites of the glass ; these are frequently coated with a thin 

 skin of haematite staining, and are the polygonal cracks to be after- 

 wards described. 



From Connor, Sandy Braes, have come several interesting varieties 

 of the rhyolite, including the ' pitchstone-porphyry ' of Portlock, 

 and amongst them a greyish-green variety (1861) with a horny- 

 looking groundmass and small porphyritic crystals. In the cracks 

 and cavities of this rock opal is not unfrequently deposited, as seen in 

 the two specimens deposited in the Dublin Museum. 



III. Microscopical Characters. 



Von Lasaulx gives the following list of minerals as occurring in 

 the Tardree rhyolites : — 'Sanidine, clinoclase, tridymite, quartz, 

 biotite, magnetite, epidote, apatite.' To this list I have only to add 

 the following : very rare pseudomorphs after hornblende, and minute 

 crystals of zircon and rutile. 



These minerals are set in a brown translucent glass, sometimes 

 perfectly pure, but generally containing ' abundance of minute 

 trichites showing all the forms figured by Zirkel. 1 Grains of 

 magnetite are also present, and minute microlites of felspar, which 

 are generally forked and frequently are mere skeletons built in a 

 negative crystal of glass. There is sometimes a very slight clearing 

 of the matrix in the immediate neighbourhood of the magnetite. 

 Occasional patches With trachytic structure are to be seen in the 



1 Zeitschr. Deutsch. geol. Gesellsch. vol. xix. (1867) pi. xiii. figs. 8 and 14, and 

 pi. xiv. fig. 2. ... 



