Lxullianite is well known, at least by name, in this country, and indeed it enjoys an international reputation among petrologists. Its general characters have been described by Bonney and Sir John Flett, while reference to the type is to be found in most standard text-books on petrology. It is, however, a rock of outstanding interest and deserves fuller description and illustration than have yet been accorded it.

The preparation by Mr. A. V. Weatherhead of a large set of thin sections for teaching purposes has given the writer an opportunity of making a more detailed examination of the micropetrology of the rock, with special reference to the manner of the replacement of the several original components.