Some time ago a specimen studded with small but well developed crystals of a sky-blue mineral, supposed to be percylite, was offered for the British Museum collection : the rarity of percylite and the excellence of the crystals on this specimen rendered a careful examination desirable. The specimen had been obtained from the Mina Beatriz, Sierra Gorda : on a large-scale chart, kindly placed at my service some years ago by Mr. George Hicks, of Newquay, Cornwall, Sierra Gorda is shown situate between the Bay of Mejillones and Caracoles, about 20 or 30 miles from the latter; longitude 69°29' W. of Greenwich, latitude 22°55' S. : at the truce of 1884 possession of this district was temporarily assigned to Chili.