Extract

An olivine-bearing dyke, about 8 feet in width, is seen cutting the foliated gabbros of Glen Ernan at a point close to the left bank of the Ernan, near the northern edge of the wood behind the mansion house of Ednaglassie, Strathdon, Aberdeenshire. The rock has a specific gravity of 3·06, and presents some interesting petrological characters which may be briefly described. It shows olivine in varying proportion from about a third to a half of its volume. The mineral is in places wonderfully fresh, is colourless, or of a faintly greenish-yellow tint, and shows lines of minute magnetite crystals, often beautifully contoured, along and parallel to its numerous fissures. At times the olivine crystals are surrounded with coronas of brown hornblende, less frequently and less continuously of biotite. The latter mineral exists in scattered patches independently of its relation to the olivine. Where the olivine has undergone a change in the direction of serpentine, it is of a darker greenish colour, and is bordered, where it meets the felspars, by narrow fringes of a greenish decomposition product. Numerous highly refracting reddish-brown points, occasionally showing crystalline outlines, are seen enclosed, oftenest in the olivine, through occasionally also in the hypersthene, and frequently give rise to a reddish brown stain along the fissures of the surrounding minerals. They are probably a decomposition product. Apatite occurs as an inclusion in the olivine, as well as in the hypersthene, but is also to be seen as an independent mineral. The hypersthene, which exists