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I. Introduction The Garabal Hill-Glen Fyne igneous complex lies just west of the head of Loch Lomond and stretches westward into Glen Fyne. The complex occupies an area of some 12½ square miles embraced by sheets 37, 38, 45 and 46 of the one-inch Geological Survey map of Scotland. It breaks through various Dalradian schists already folded and regionally metamorphosed and these suffered thermal metamorphism when the rocks of the complex were intruded. The age of the complex is generally accepted as Lower Old Red Sandstone, owing to its close resemblance to other igneous masses of that age in Scotland. The earliest account of the area is given by Dakyns and Teall (1892) in a paper which has since become classical. The boundaries of the massif were accurately delineated, some of the rock types were mapped, briefly described and chemically analysed, and the hypothesis put forward that they had been derived from some parent magma by a process of differentiation operating at depth. A description of part of the western portion of the mass and of the thermal metamorphism induced there in the surrounding schists was published in the Cowal Geological Survey memoir (1897, pp. 96–101) and in the mid-Argyll memoir (1905, pp. 92–3). Finally, a paper by Wyllie and Scott appeared in 1913, dealing mainly with the ultrabasic rocks of the complex but including some data on the basic and intermediate rocks. The variation in the ultrabasic and dioritic rocks was ascribed to local variation

The Caledonian plutonic rocks of Western Scotland form a well-defined group, ranging from ultrabasic to acid types, which have been studied in detail by a number of petrologists. They form intrusions of varying size, usually composite and frequently, at least, having the habit of ring complexes. The rock types include peridotites and pyroxenites, gabbros, hornblendite and appinite, appinitic diorites, pyroxene-mica diorites, hornblende-biotite diorite, various hornblende-biotite grandiorites, biotite adamellite and muscovite-biotite adamellite, while the final stage is represented by numerous veins of aplite and rare pegmatites.