Silicated calcareous concretions occur in paragneisses and mica schists of Archean age near Thunder Lake in northwestern Ontario. Anorthitic plagioclase, quartz, and hornblende are the chief constituents, and clinozoisite and garnet are the most significant minor components. The high lime content shown by chemical analysis, the peculiar mineral assemblage, the nodular structure of these bodies, and their relation to the containing beds leave no doubt of their concretionary origin.

Similar metaconcretions have rarely been reported from Archean rocks. Their importance in support of the uniformitarian principle is discussed.

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