The CNOC Nan Cuilean Area of the Ben Loyal Igneous Complex

Abstract

I. I ntroduction T he Ben Loyal Igneous Complex forms an impressive group of mountains situated in northern Sutherland about six miles south of Tongue. The rock types include three distinct varieties of syenites, composing respectively the Ben Loyal range, Cnoc nan Cuilean and Ben Stumanadh, together with related syenites, aplites and pegmatites which constitute numbers of dykes, sheets and veins. The only previous work on this area of which an account has been published was carried out by the officers of the Geological Survey during the mapping of sheets 108 and 114 (Scotland). In the Geological Survey memoir on Central Sutherland (1931, pp. 174—9), H. H. Read gives a brief description of the field occurrence and petrology of the syenites; the rocks are described as unfoliated syenites and nordmarkites and their general petrographic characters are noted. The evident similarity of the Ben Loyal rocks to those of the Loch Ailsh intrusion is demonstrated both mineralogically and with the aid of chemical analyses. For more general references see J. Phemister (1936, p. 57) and E. B. Bailey and 0. Holtedahl (1938, p. 35). During the investigation here recorded, a field study was made of the Cnoc nan Cuilean and the Ben Loyal intrusions, and, for purposes of comparison, several traverses were made over the Ben Stumanadh mass. The primary purpose of this paper is to describe and interpret the petrography of the Cnoc nan Cuilean mass. This intrusion presents features not to be seen in the other two masses, namely, the development