III.—Summary of Recent Research on the Pre-Tertiary Geology of the Northern Highlands

Abstract

I. The Lewisian Gneiss The eclogites and their retrograde representatives, garnet-amphibolites, which occur in the Glenelg area were studied in detail by A. R. Alderman (1936) who showed that these rocks occur in bands which course N.N.E.-S.S.W. with the paragneisses of the locality. The eclogitic rocks have the composition of basalts and represent minor intrusions, and their association with the paragneisses is fortuitous according to Alderman. The retrograde metamorphism and the dispersion of these basic rocks among the banded gneisses of the region are due to the post-eclogite permeation by pegmatitic material. From the same general area Tilley (1936) described still more basic granulites which are definitely members of the paragneiss series. The rocks are granular aggregates of fayalite, grunerite, garnet, magnetite, hedenbergite in varying proportions, the variety in which fayalite is a dominant constituent being called eulysite. In composition they correspond to ferruginous sediments such as greenalite chert and siliceous ironstones. Two less specialised papers on areas of the Lewisian include (1) description of the Harris paragneisses and metagabbro-anorthosite suite by C. F. Davidson (1943), and (2) description of the complex lithology of the gneiss in the Torridon area together with discussion of the metamorphic history of the Lewisian of the mainland by J. Sutton and J. Watson (1951). Davidson showed that the paragneisses represent a series of interbanded shallow water sediments of siliceous, argillaceous and calcareous composition. These with associated minor basic and ultrabasic intrusions and larger masses of gabbro and anorthosite suffered deep regional metamorphism with the This 250-word extract was created in the absence of an abstract