There is an extensive but widely scattered literature on the subject of coronas and reaction rims. The observations of many writers are summarized here, and it is shown that further chemical study and close observation of geological relations are essential to the solution of the corona problem. The writer supplies new chemical analyses of the corona-bearing rock of Gore Mountain, Adirondacks, and of the garnets in it, and a description and analysis of a recently discovered coronite from eastern Quebec. The idea is developed that the fundamental factor in corona formation may be the instability of olivine under thermal metamorphism, leading to the discharge of iron and magnesium ions which react with plagioclase, the conditions being those of an open system.

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