The Age of the Sarn Granite

Abstract

The age of the granitic and gneissose rocks near Sarn, Rhôs Hirwaen, and Meillionydd in south-western Lleyn has long been a subject of difference of geological opinion. The granite and its gneissic associates were originally mapped together by the Geological Survey as “syenite”, and were generally known as the “Rhôs Hirwaen syenite”; this was considered by Ramsay to be of Palæozoic age. Later, Hicks visited Lleyn, and among the numerous rocks in that district which, in 1878, he claimed to be of Pre-Cambrian age was the Rhôs Hirwaen “syenite”, which he called a metamorphic Dimetian granitoidite. Tawney soon afterwards examined the geological relationships of the rocks mentioned by Hicks and came to the conclusion that “of all the areas of rock in the Lleyn promontory which Dr. Hicks claims as Pre-Cambrian, this patch of metamorphic rocks near Sarn [that is, the Rhôs Hirwaen “syenite”] seems to be the only one with any claim. It is, however, unfortunate that we have not been able to find any fragments of it inclosed in Cambrian beds” (1883, p. 68). Tawney's opinion was based on the examination of the junction of the granite and Ordovician sediments in a quarry near a fine cromlech at the north-eastern margin of the granite mass, which he calls “the quarry by Amwlch Lodge”. As the cottage is now shown on the six-inch map as “Mountain Cottage” the exposure will be referred to in future as “Mountain Cottage Quarry” (Fig. 1). Tawney's account shows that he had considerable difficulty