The occurrence of abundant and even conspicuous metacrysts of magnetite in the iron formation of the Nemo region of the Black Hills attracted the writer’s attention to magnetite as a metacryst in metamor-phic rocks. Magnetite is well known to have a strong crystallizing power and it corresponds well with Becke’s1 statement that minerals in schists with compact molecular arrangement are most likely to take on crystal form, but Becke did not list magnetite in his series of form development. Grubenmann omitted magnetite in 1904, but Grubenmann and Niggli2 in 1924 give it last in a list of porphyroblastic minerals. Magnetite is not given by Leith and Mead3 among the common idiomorphic minerals in schists but their list is indicated as not complete. Grout4 lists magnetite among the minerals common as metacrysts.

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