Two interesting rock types result from calichification and alteration of the weathered surface of bedrock limestone adjacent to shallow saline lakes on Isla Mujeres, Quintana Roo, Mexico.

Jet-black micritic limestone is produced on the periphery of the saline lakes, apparently as the combined result of calichification, blue-green algal penetrations, and sulfate-reducing bacterial action. Micrite layers are added by calichification. The black color is attributed to finely disseminated organic material and iron sulfide(?) produced by bacteria. Bacterial action not only may be partly responsible for the black color of the limestone, but also may account for the small amount

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of calcium sulfate found in the lake deposits. Similar black, algal, filament-bearing micrite, found as rounded pebbles on Caribbean beaches of Isla Mujeres, most likely was produced in a similar environment at a lower stand of sea level and worked shoreward from eroded, submerged outcrops.

Hard caliche breccias crop out in several places adjacent to the salt flats. These rocks contain both angular fragments of bedrock and caliche and rounded, pisolitelike structures. Similar breccias are found on low rocky sea cliffs just above the surf zone of Isla Mujeres as well as on other islands in that vicinity.

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