The prevailing view is that tufa deposition at Mono Lake, Calif., is predominantly inorganic; it is here shown that algae probably initiate much of the CaCO 3 precipitation from the highly alkaline waters of the lake. Two exposed pinnacles were studied in detail; one is dry and the other has a summit spring. A calcareous blue-green algal mat covers the sides of the summit-spring pinnacle beneath the spring water, and is partly embedded in a thin layer of dense lithoid tufa sheathing the porous lithoid tufa which forms the bulk of the pinnacle. The porous lithoid tufa consists about equally of microcrystalline calcite (aragonite in the dry pinnacle) rife with algal remains, and coarse sparry calcite, also regarded as partly organically deposited.

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