The mineral cummingtonite occurs abundantly in the Homestake mine, Lead, South Dakota, and likewise to the north of this locality. It also occurs near Rochford, twenty miles southwest of Lead. The cummingtonite is developed in iron-magnesium carbonate horizons in pre-Cambrian schists. It has been described1 as due to regional metamorphism and to direct reaction between the original iron-magnesium carbonate (sideroplesite) grains and associated grains of sedimentary quartz. However, recent mapping by the geological department of the Homestake Mining Company has shown cummingtonite to be developed only locally and to be associated with original unaltered quartz grains. It is closely related to ore minerals and shows none of the fracturing that might be expected if it had been subjected to regional stresses. In view of this, an alternative hypothesis is that the cummingtonite was formed by the action of hot silica-bearing solutions on sideroplesite. Some of the cummingtonite replaces minerals of known hydrothermal origin.

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