On the Eruptive Rocks in the Neighbourhood of Sarn, Caernarvonshire

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Abstract
T he district to be treated lies to the west and south of the village of Sarn, near the south-western extremity of Caernarvonshire. The eruptive rocks there exposed, excluding outlying patches, occupy an area of irregular shape, which extends about 5¾ miles from north to south, and has a greatest breadth of about 2¼ miles. This part of the country has received but brief notice from Sir A. Ramsay in his memoir on the “Geology of North Wales”, and from Dr. Hicks, who claims a portion of the area for his Pre-Cambrian systems. Several specimens from the district have been described by Professor Bonney and the late Mr. Tawney, and the latter has also made a few observations on the field-relations of some of the rocks; but with these exceptions we have no published information about the western part of the Lleyn peninsula, though there are probably few districts of equal size in Britain where so many interesting rock-types are to be met with. The rocks will be discussed in the following order :—Granite and Gneissie Granite ; Gabbro, Diorite, and Gneissic Diorite; Diabase; Hornblende-Diabase; Hornblende-Picrite; and Dolerite. Some of these, however, are but little developed, and, for most purposes, the rocks of the district may be divided into two groups—an acidic, developed in the north and west, and a basic and intermediate in the east and south. Mr. Tawney and Dr. Hicks have pointed out that the map of the Geological Survey

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