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
