II.—The Archæan Rocks of the Rodil District, South Harris, Outer Hebrides

Abstract

The Archæan rocks of the southernmost part of the parish of Harris in the Outer Hebrides have long been known to include not only a wide variety of gneisses of igneous origin, but also a group of paragneisses which, along with presumably contemporaneous formations in Glenelg, Tiree, Iona and elsewhere, probably represent the most ancient sediments in Britain. Apart from early references by Macculloch (1819) and Heddle (1878, 1888, 1901), for practically all our present information on these gneisses we are indebted to Jehu and Craig (1927), who have made a reconnaissance survey over the whole chain of the Outer Hebrides and whose work forms the standard account of the geology of this region.The present paper gives a description of the gneisses around the village of Rodil, a district of somewhat complex petrography and metamorphic history. The paragneisses present include crystalline limestones, calc-silicate rocks, garnet-kyanite-gneisses, graphite-schists, quartzites, and other schistose and gneissose rocks less easily described by short titles. These sediments have been intruded by sills or sheets of basic to ultrabasic composition, now represented by gneisses of the composition of eclogite, garnet-amphibolite, amphibolite, peridotite, pyroxenite, saxonite, websterite, and a series of rocks referred to under the portmanteau name of metagabbro—types akin to pyroxene-granulites and basic charnockites, often garnet-bearing, but locally with pyroxene and garnet replaced to a varying extent by hornblende and biotite. There is also present a large mass of anorthosite-gneiss bordered by metagabbros, both the feldspathic and basic rocks bearing frequent schlieren of eclogite, garnet-amphibolite, and amphibolite. The complex is traversed locally by acid and basic pegmatite dykes and sheets.