以下为句子列表：
英文: This hematite is gray, however, because the grain size is larger.) The layering of these distinct rock types indicates that the Huronian glaciations occurred precisely when atmospheric oxygen levels first rose.
中文: 这些层次分明的岩种，暗示著休伦冰期恰好发生于大气中的氧气浓度初升之际。
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