Precambrian formations of Northwestern North America contain no fossil assemblages suitable for ecologic analysis. The closest approximations to associations are found in stromatolite biostromes and reefs of the Snowy Range and Belt series, in which stony algae were the dominant life forms. Snowy Range stromatolites were produced in quiet, shallow marine waters by algae intolerant of clastic sediment. Builders of Belt stromatolites were tolerant and very hardy, since they thrived in shallows subject to disturbance by storms, floods, lava flows, and periodic desiccation. Reef stromatolites show great variation, apparently in response to varied conditions of light and water.