GREAT masses of pyritic minerals include some of the most valuable and interesting of known ore deposits. These great ore bodies are of high economic importance, for, though their intrinsic value is low, their 'vast bulk enables them to be often worked by simple quarrying; they are full of scientific interest, from the many problems connected with their formation; and they are famous historically, as, owing to the ease with which the copper can usually be extracted from them, they have yielded supplies of that metal since pre-historic times.