OCCURRENCES OE TACHYLYTE. 305 



magnetite of the rock, the glass being, for a tachylyte, unusually 

 clear. 



The sections also show the passage of the rock into a stony con- 

 dition by the dense accumulation of spherulites, though alteration 

 has here gone considerably farther than in the vitreous portions. 

 Altogether, this Quiraing fragment presents, in its wealth of detail, 

 the most beautiful example of basalt-glass with which I am ac- 

 quainted (PI. XI. hg. 3). 



If we find, however, that this instance exhibits certain phases of 

 alteration, the next example has advanced several stages on the 

 downward path. At an elevated point of the highroad from New- 

 castle, County Down, west of Bryansford and near the park of 

 Tollymore, a columnar basalt-dyke traverses the Ordovician strata 

 of the district, and is bounded on both surfaces by decomposing 

 tachylyte. This selvage, however, must have at one time resem- 

 bled closely the rock of the Quiraing, although the glassy interspaces 

 are now green in section, and the spherulites a ruddy brown. 

 Cumulites are observable in the thinnest portion of the slide, the 

 magnetite is aggregated into little cubes, and the fibrous structure 

 of the spherulites still remains, the characteristic black crosses with 

 polarized light being easily apparent. A rude perlitic structure 

 and traces of banding traverse the rock ; and a few corroded 

 porphyritic felspars have been seized on as centres of devitrification. 

 The numerous vesicles, on the other hand, seem to have had a very 

 partial connexion with the arrangement of the crystallizing par- 

 ticles (PL XI. fig. 4). 



The basalt at the centre of the dyke contains some biotite and a 

 fair residuum of glassy matter ; like the rock of Ardtun headland, it 

 has closer relations with the augite-andesites than with the basalts 

 rich in olivine. 



The extent to which alteration has proceeded in this last case, 

 without the fundamental characters of the rock becoming obscured, 

 and the corresponding retention of original structure among the 

 most ancient and devitrified of acid glasses, leads one to inquire 

 whether tachylytes may not be identified among the relics of our 

 older volcanic areas. Already Mr. Rutley* has given detailed 

 evidence that the rocks of St. Minver, Cornwall, include vesicular 

 andesite-glass in a state of considerable alteration. As has been 

 often pointed out f, the surface-products and other glassy portions 

 of ancient and denuded lavas are frequently to be recovered among 

 the ashes and detrital deposits formed during the eruptive period. 

 Thus at Snead, near Bishop's Castle, on the outskirts of the Corn- 

 don volcano, a tuff of Ordovician age occurs, containing numerous 

 black and blue-black fragments which at once recall the glass-particles 

 of more recent areas. The palagonite-tuffs of Sicily, the Kaiser- 

 stuhl, and Iceland, or the glassy tuff of Hilzingen in the Hegau, 

 may be cited as examples of such deposits of later age. The black 



* Quart. Joum. Geol. Soc. vol. xlii. (1886), p. 392. 

 t E. g. Jukes, Journ. Geol. Soc. Dublin, vol. viii. p. 32. 



