On some additional Occurrences of Tachylyte

Abstract

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 Bryansford, 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 microscopic characters of the tachylyte of the Quiraing in Skye. The Duke of Argyll, in his classic description of the leaf-beds of Ardtun in Mull†, 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