The Picrite-Teschenite Sill of Lugar (Ayrshire)

Abstract

I. Introduction. The association of teschenite and ultrabasic rocks (picrite and peridotite) in a single rock-body has now been established for several occurrences in the lowlands of Scotland. The Barnton occurrence, near Edinburgh, has been described by Sir Archibald Geikie,1 by Mr. J. Henderson & Mr. J. G. Goodchild, and by Mr. H. W. Monckton. At Blackburn, near Bathgate, occurs a picrite, which has been described by the first-named writer as a lava, but has recently been shown to be intrusive and associated with teschenite by the officers of the Geological Survey of Scotland. The famous picrite of Inchcolm, in the Firth of Forth, is well known from the descriptions of several observers. Dr. R. Campbell and Mr A. Stenhouse, however, in a recent detailed investigation of the island, have shown that at both the upper and lower contacts the picrite passes into teschenite. On the west coast, Dr. J. D. Falconer has described a picrite-teschenite sill at Ardrossan, intrusive into the Carboniferous Limestone Series: here again the ultrabasic rock occupies the central part of the mass. Recently the late R. Boyle drew attention to still another occurrence at Lugar, near Old Cumnock (Ayrshire). He described the passage of dolerite and basalt [teschenite] through doleritic picrite [theralite] to ‘segregated’ masses of peridotite or picrite. The ultrabasic rock occurs in the central parts of the mass, find passes gradually to less basic varieties towards both upper and lower contacts. A picrite associated with teschenite has-been discovered by Mr. E. M. Anderson