II.—On Scottish Rocks containing Orthite

Extract

From America and from many different localities on the continent of Europe the occurrence of Orthite or Allanite as an accessory mineral in crystalline rocks of different kinds has been frequently described. But so far it does not seem that its occurrence in Great Britain has been recognized, or at any rate I have failed to find any mention of it. Quite recently two instances of Scottish orthitebearing rocks have come under my notice which I wish to describe briefly in the present communication. Both are in acid holocrystalline rocks—granites and gneisses—and in both it is associated with the closely allied mineral epidote, as is usually the case. For specimens of the granite of Fell Hill I am indebted to Mr. James More, jun., M.Inst.C.E., Edinburgh, who sent sections of this rock for microscopic examination to Professor James Geikie, Edinburgh University. Although only two in number, the sections contain at least eight recognizable crystals of orthite, besides smaller pieces about the identity of which there might be room for doubt, and some of these are fortunately so cut as to render possible not only a certain identification of the mineral, but also a fairly complete account of its optical properties.