188	PRECIOUS STONES.
	cases to be a product of thermo- or dynamo-metamorphism, or of a combination of both. Ruby.—The most important Ruby mines are in Burma, around Mogok to the east of the Irrawaddy, and north-north­east of Mandalay, and specimens from these mines most nearly approach the ideal in colour. A smaller area of Ruby-bearing limestone is found at Sagyin Hills, north of Man­dalay. Professor Judd, in the article dealing specially with the Ruby of Burma (Brown and Judd, Phil. Trans., Vol. CLXXXVIIa.), ascribes the limestone in which Rubies occur to a decomposition of the lime Felspar contained in the basic gneisses. There is a peculiar feature about the interrelation of the limestone and gneisses, for they show a considerable amount of interbedding. The limestones are not sharply marked off from the gneisses, but merge gradually into them, and Mr. Barrington Brown describes them as having their dips conformable to the contortions of the gneiss in all cases. Dr. Noetling, of the Indian Survey, regards tbe igneous rocks as having been intruded into the limestone in a molten state, and holds that the Rubies and other minerals contained in the limestone are the product of contact or thermo-metamorphism. The role played by heated water under pressure would seem to enable the formation of a gneiss to occur largely out of the material of the rock around, and thus to replace this rock, the semi-fluid magma at the same time throwing out projections and enveloping portions of the country rock. It is in the limestones alone, and in the debris resulting from their weathering, that the Rubies are found. This debris consists of a brownish clayey material, and it fills the crevices and " shaks " of the limestone and covers the sides and bottoms of many of the valleys. The