304 ME. G. A. J. COLE ON SOME ADDITIONAL 



the quarry a sharply- defined little dyke or vein, perhaps connected 

 with the other below the surface, and 2^ inches wide at the most, 

 was similarly bounded by black glass. 



In section this last-named tachylyte appears brown, but its nar- 

 rowness diminishes its interest. The development of felspars in the 

 adjacent basalt is, however, well displayed, the colourless prisms 

 being often incomplete or bifurcated at the ends. This basalt is of 

 normal character, with porphyritic olivines, a fact that may in itseK 

 explain, when ordinary temperatures are concerned, the small de- 

 velopment of the glass. The surrounding rock is a rudely columnar 

 hornblende-mica-porphyrite, with inclusions of mica-slate, and 

 shows in section a very thin film, as if of partial fusion, at its 

 junction with the invading basalt *. 



I am indebted, as already mentioned, to Mr. A. W. Dymond, of 

 the Royal School of Mines, for a specimen of tachylyte from the 

 Quiraing in Skye. Although this fragment was found upon the 

 talus, it probably formed part of the layer described and analyzed 

 by Prof. Heddle f, and this fact must be my excuse for dealing 

 with a rock with which I have no acquaintance in the field. 



The glass in its present condition is easily scratched with a knife, 

 and is, in fact, as its analysis sufficiently indicates, verging on 

 palagonite. It retains, however, when viewed in microscopic 

 section, all the delicate structure produced by devitrification during 

 cooling. The matrix is of a yellow-brown tint, and includes nume- 

 rous small spherulitic aggregations, about 1 millimetre in diameter. 

 The first-formed fibres of these are frequently arranged in sheaves, 

 recalling the " chiasmolites " of Krukenberg J, which are in this 

 case converted by subsequent additions from without into double or 

 single spherulites and axiolites. In comparison with their rich 

 brown colour, the surrounding glass looks almost grey. The steam- 

 vesicles in this rock seem to have had no influence whatever on the 

 development of the spherulites. 



The residual glass abounds in transparent globulites, and in 

 spherical groups of these minute bodies, forming the cumulites of 

 Vogelsang. There are also little bunches of dark crystallite-fibres, 

 grouped so as to form right-angled figures with hollow sides. 

 These, together with more defined crystals, probably represent the 



* Through the kindness of Mr. F. H. Butler, M.A., I am able to record here 

 the following occurrences of tachylyte, which were noticed by himself and Mr. 

 P. F. Kendall, during a visit to the Isle of Mull. A columnar dyke below a 

 waterfall in the Tobermory Burn, about three feet across, and a fine-grained 

 columnar dyke at Eudha nan Gall, are bordered by basalt-glass. Another 

 basalt, near the Erray Burn, west -north- west of this last point, and containing 

 porphyritic felspars, has also vitreous selvages. 



t Min. Mag. vol. v. p. 8. In the errata of the same volume, Prof. Heddle 

 directs attention to the old term " Gallinase." It is of interest to find that in 

 Buffon's • Histoire Naturelle' (edition of 1801 &c., vol. ix. p. 349), Gallinace 

 is carefully described as a black semi-transparent glass occurring in Iceland, 

 Etna, Peru, &c., amid volcanic matter, in a manner that suggests its identity 

 with the sideromelane and palagonite of von Waltershausen. The name is 

 derived from the Gallinazo, a black carrion-bird of the Andes. 



X Mikrographie der Grlasbasalte von Hawaii (Tiibingen, 1877), p. 8. 



