The choir and chancel do not stand on the island's granite core but on a platform formed by three crypts, with the massive columns of the Crypte des Groseiliers doing most of the work.
The island is made of volcanic rock material.
mnli hypothesis: The island is made of volcanic rock material. premise: The choir and chancel do not stand on the island's granite core but on a platform formed by three crypts, with the massive columns of the Crypte des Groseiliers doing most of the work.