Investigation of a Pennsylvanian limestone unit exposed near Molas Pass, southwestern San Juan Mountains, Colorado, reveals a distribution of carbonate materials similar to those on some modern banks of south Florida. Local thickenings of an interval containing the problematical red alga Komia suggest bank-like accumulation at shallow sites which were favorable especially for the prolific growth of this organism. The size and twig-like nature of Komia and its common association with pelleted micrite matrix suggest a comparison with the modern alga Goniolithon which inhabits the Florida banks. Such comparison leads to the interpretation of a very shallow-water environment for the Pennsylvanian counterpart. Micritic matrix is replaced by sparry calcite cement near the fring of the Komia banks and demonstrates the slightly more agitated conditions of the seaward margin where carbonate mud was removed selectively from around Komia grains.

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