The Propylites of the Western Isle of Scotland, and their Relation to the Andesites and Diorites of the District

Abstract

I. I ntroduction . There exists in the Western Isles of Scotland a great series of lavas which, for the most part, underlie the ophitic olivine-basalts, and constitute the oldest of the ejections of the great Tertiary volcanoes of that district. These rocks were distinguished by me in 1874 under the old English field-name of “Felstones,” and it was stated that they “vary in colour from black, through various shades of green and grey to white; but in almost all cases their surfaces acquire a white crust in consequence of weathering action” In attempting, at that date, to define more exactly the characters of these lavas, by studying them microscopically in thin sections, I was confronted by two difficulties. In the first place, it soon be became manifest that these more acid lavas of the Western Isles include a great variety of types--differing widely from one another in mineralogical constitution and in structure; and in the second place it was found that the minerals of which those rocks were built up were in a remarkably altered condition. In both these respects, the “felstones” present a very striking contrast to the overlying series of ophitic olivine-basalts. The latter, as I have shown, are remarkable for their uniformity of composition and character, and, except where influenced by surface-agencies, seldom present any great signs of alteration; they are indeed, as a rule, singularly fresh and unchanged in their appearance. But in the “felstones” which underlie the basalts the most extreme metamorphism is seen to have