II.—Some New Data on “Kylitic” Sills and Associated Picrites in Ayrshire, Scotland

Synopsis The definition of the name “kylite” is reviewed and it is suggested that its usage should be discontinued. What is now emphasised as the criterion of most importance in distinguishing “kylites” from crinanites and teschenites is the development of a strongly zoned purplish pink augite in advance of feldspar. Basic alkaline doleritic rocks in which the augite develops in this manner could be referred to simply as kylitic. Specimens were systematically collected from the picritic sill near Dalmellington which is kylitic in type. Rocks from different levels in this sill have been chemically analysed, and also unzoned olivine (Mg74.4) and zoned augite. A vein (and the augite from it) in the kylitic sill near Troon, known as the Hillhouse Quarry Sill, have also been chemically analysed. The chemical compositions of all these analysed rocks and of the analysed augites are compared with appropriate data from, in particular, the sills of the Shiant Isles and Lugar, and the Black Jack Sill in New South Wales. Some relationships of kylitic rocks with the differentiation trends in these sills are briefly discussed with the aid of variation diagrams. On the basis of the available evidence it is inferred that in the olivine-rich Dalmellington Sill, olivine has accumulated prior to intrusion and that subsequent “flowage differentiation” has apparently been inoperative.