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



23. On some additional Occurrences of Tachtltte. 

 By Grenyille A. J. Cole, Esq., P.G.S. (Read February 29, 1888.) 



[Plate XI.] 



Since the spring of 1883, when I had the honour of being associated 

 with Prof. Judd in a paper on the Basalt-glass of the Western Isles 

 of Scotland*, a few additional occurrences of tachylyte in the 

 British Isles have come under my notice in the field. Specimens 

 have been thus collected from Ardtun in Mull, Kilmelfort in Argyll, 

 from near Bryausford, County Down, in Ireland, and among certain 

 older rocks of the Welsh border. I am also enabled, by the kindness 

 of my friend Mr. A. W. Dymond, to give an account of the micro- 

 scopic characters of the tachylyte of the Quiraing in Skye. 



The Duke of Argyll, in his classic description of the leaf-beds of 

 Ardtun in Mullt, refers to a glassy layer between two of the 

 basaltic masses of the headland ; and the same occurrence was 

 mentioned by Mr. Koch during the discussion on the paper dealing 

 with Scottish basalt-glass. Mr. J. Starkie Gardner, who called my 

 attention to the rock upon the ground itself, has described and 

 figured the course of an intrusive sheet that forms a very conspicuous 

 feature at Ardtun %- At its most accessible portion it is about 

 8 ft. thick, and in the centre still retains, as shown by microscopic 

 examination, a considerable amount of colourless glassy matter. In 

 this matrix cumulites and belonites are developed; plagioclase 

 felspar is abundant, interspersed with brown prismatic augite, while 

 the magnetite is collected into crystals that are often well defined. 

 On a sea-face so exposed the rock has naturally suffered, and the 

 black lustrous specks visible in it to the naked eye prove to be soft 

 products of alteration §. It does not appear, however, that they are 

 pseudomorphs after olivine, a mineral which is in this case rare, if not 

 altogether absent. On the other hand, the few specks of quartz 

 that occur are clearly of secondary origin. 



Theuj)per and lower surfaces of this intrusive sheet have, by rapid 

 cooling against the surrounding basalt-flows, become coated with a 

 black glassy selvage, which is seldom more than an inch in thickness. 

 This tachylyte adheres more firmly to the contact-rocks than to the 

 mass from which it was developed; but microscopic examination 

 opposes the idea that an intermingling through fusion has taken 

 place along the planes of junction. The crystals of the overlying 

 basalt are, indeed, seen to be sharply broken through, and abut un- 

 altered against the intrusive glass. 



The tachylyte itself is fairly fresh, though traversed by the 

 numerous joint-planes characteristic of this class of rock. Its 



* Quart. Journ. Geol. Soc. vol. xxxix. p. 444. 

 t Ibid. vol. vii. (1851), p. 94. 

 + Ibid. vol. xliii. (1887), pp. 271, 272. 



§ Compare the chlorophgeite of MaccuUoch, ' Western Islands of Scotland,' 

 vol. i. p. 504. 



