The breccia occupying most of St. Helen's Island and Ile Ronde was originally considered to be sedimentary, later volcanic, and is now attributed to diatreme action. It contains fragments of all local formations including the Precambrian basement and also Devonian limestone blocks.Recent excavations on Ile Ronde have shown fracture zones in the breccia filled exclusively with fragments of the adjacent Utica shale. More important is a new type of breccia consisting of sub-rounded masses of Precambrian quartzites and other rocks (but no anorthosite) held in a matrix of olivine diabase of Monteregian affinities.

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