Carbon-rich rocks more than 2.7 billion years old, from the Soudan Iron Formation of northeastern Minnesota, contain pyrite balls in which are found microstructures of possible biologic origin. The C(13)/C(12) and S(34)/S(32) ratios and minor element composition of this rock are consistent with such an interpretation. Hydrocarbons of presumed vital origin are also present, although it is not certain that they are endemic to the rock.
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Cloud et al. (1965) studied this question.
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