IX. On the Occurrence of Tachylyte at Loch Screden, Mull

Tachylyte is the black, bottle-glass selvage which is occasionally seen upon the contact surfaces of basaltic dykes. In a paper upon the tachylytes of the Western Islands of Scotland, published in the Quarterly Journal of the Geological Society for April, 1883, by Professors Judd and Cole, these authors give what is termed a detailed “History of previous opinions on the subject.” In this, after showing that a variety of terms had been applied to this selvage upon the Continent, they say “Zirkel and others describe different varieties of basalt-glass under the name of tachylite, and by the same designation the Scottish examples were referred to by one of us in 1875.” Under a second heading, “Distribution and mode of occurrence,” due credit is given to Dr. M'Culloch as having been the original discoverer of two, out of what are termed the “five known localities.” Under the above headings no notice is, however, taken of the observation, and very full, interesting, and accurate description, of the occurrence of this substance by Prof. Archibald Geikie at Ben Tigh in Eigg,—published in his admirable paper on the geology of Eigg in the Quarterly Journal of the Geological Society in 1871; nor of a chronicle of the occurrence of the same substance at the Quiraing in Skye, as submitted to the Mineralogical Society in 1880 by the present writer, and published in the Mineralogical Magazine. In this latter paper the writer showed, by analysis of both, that the composition of the tachylyte, and that This 250-word extract was created in the absence of an abstract