On the Basalt-Glass (Tachylyte) of the Western Isles of Scotland

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Abstract
In a previous paper* it has been pointed out that the Tertiary volcanic rocks of the Western Isles of Scotland offer beautiful examples of materials of every variety of composition, from the most acid to the most basic, and of every type of structure, from the holocrystalline to the vitreous. The detailed description of these varieties of volcanic rocks was reserved for a future occasion ; and in the present paper we propose to give the first of such a series of descriptions. As the more acid vitreous rocks have during recent years been discussed in considerable detail in numerous papers read before this Society, it may not be inappropriate to direct attention to the rare but equally interesting glasses of basic composition, which have, up to the present time, received far less notice in this place. The studies on which this paper is based have been carried on in the Geological Laboratory of the Normal School of Science and Royal School of Mines. 1. History of Previous Opinion on the Subject . By the older writers on petrography rocks of the kind of which we now treat appear to have been classed as “pitchstones.” Jameson and the other followers of Werner, who endeavoured to introduce the precision of nomenclature and the exact methods of their master into the study of British rocks—though they recognized these materials, not as minerals but as rocks—do not seem to have discerned the difference between the acid and basic

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