Some Scottish granite-gneiss contacts and their interpretation

Abstract

The purpose of this paper is the description of some specimens illustrating contacts between granitic and gneissose rocks, and the interpretation of the phenomena which they exhibit. The specimens in question were collected by the late Professor Heddle, and were presented by him to the Royal, Scottish Museum; they have been exhibited in the Geological Department of this Museum for the last twenty years, but have not previously been described. Contacts of this kind are not too often seen in the field, and are rarely if ever within easy access of students; it seems desirable, therefore, that the existence of these superb specimens should be placed on record. Specimen No. 1. Locality. —The specimen is from Balmoral, Aberdeenshire; as regards the precise locality no information is forthcoming. Description. —The specimen is a large slab, polished on one side, and measuring 2 feet 4 inches (greatest height) by 2 feet 6 inches (greatest width). It is illustrated in Plate XXXIII., a twelve-inch rule being included in the photograph in order to give the scale. The specimen consists of a banded gneiss showing a single large unsymmetrical fold upon which minor contortions and puckerings are superposed. One limb of the fold is abruptly truncated by a mass of gneissose granite which shows pronounced foliation parallel to the surface of contact. On the polished face of the stone the line of contact between granite and gneiss is perfectly sharp. The gneiss consists of a succession of more or less sharply defined bands,