An igneous dyke in the quartz-banakite of Bangly Quarry near Haddington

Abstract

Bangly Quarry is situated about two miles to the north-west of Haddington, and is well known for the fine porphyritic quartz-banakite that occurs there; and also for a so-called very fine porphyritic trachyte carrying large crystals of sanidine up to two inches in length. It is thus described in the Memoir of the Geological Survey for East Lothian, 1910, p. 79: ‶The finest example of a porphyritic trachyte in the Garleton area is met with in the Bangly or Silver Hill Quarry. . . . It belongs to the special group of quartz-banakites, and most of the normal phenocrysts are of plagioclase; but these are associated, in the large quarry, with sanidine crystals which are often two inches long, and in many cases conspicuously twinned according to the Carlsbad law. This rock is probably the finest example of a porphyritic trachyte in the British Isles; yet so local is the development of the late formed giant phenocrysts, that at the west end of the same quarry they have almost entirely disappeared.″ The face of Bangly Quarry is nearly 80 feet high, and looks towards the north. The rock is dark red-brown in colour, and the jointing is in smooth vertical planes, except at one part near the east end. At this place a vertical dyke (Plate XL., Fig. 1), about 12 feet wide, cuts through the whole thickness of the rock, in a nearly north-and-south direction. The jointing of the dyke is conspicuous and horizontal, and there is a chilled