On the Plutonic Rocks of Garabal Hill and Meall Breac

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Abstract
The schists of the Southern Highlands of Scotland have been traversed in many places by igneous rocks. Sills and dykes are widely distributed throughout the district, and here and there, as for example in Glen Tilt, Glen Lednock, and near the head of Loch Lomond, extensive areas are composed of plutonic rocks. Our object in this communication is to describe some of the phenomena which may be observed in the last-mentioned locality. The plutonic rocks in question form the belt of high ground stretching in a south-west direction from Inverarnan. They vary considerably in chemical and mineralogical composition. Different parts of the mass can be proved by the phenomena of veins and inclusions to belong to slightly different periods, but regarded as a whole it must evidently be referred to one geological epoch. Whenever two portions in juxtaposition are seen to differ in age, the more acid, so far as our observations go, is invariably the younger. Thus the veins are more acid and the inclusions more basic than the material surrounding them. Rather more than half a mile above Inverarnan tonalite (quartz-diorite) appears in the bed of the Arnan and on each side of the stream. From this point it extends in a south-westerly direction for about miles, as shown on the accompanying map, and possesses a fairly uniform character. Garabal Hill, south-west of Inverarnan and immediately south of the place where tonalite appears in the Arnan valley, is composed of diorite. In some places, as, for instance

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