"Molar-tooth" structures in the Belt Supergroup are complexes of interconnecting, thin sheets and small spheroids, composed of uniform, blocky 5-15 mm calcite crystals that cut fine-grained clayey carbonate layers. Bauerman (1885) likened the network of sheets to the corrugated surface of an elephant molar tooth. "Molar-tooth" structure described here comes from the Chamberlain and Helena formations of the Middle Proterozoic Belt Supergroup, but the structure occurs in other silty, clayey limestone and dolomite of the Belt and in Middle and Late Proterozoic rocks elsewhere in the world. Previous investigators have applied the term "molar-tooth" to several kinds of structures and attributed them to a variety of physical, biological, and chemical processes. We adhere to Bauerman's (1885) original citation of "molar-tooth" and limit our definition to blobs, vertical ribbons, and horizontal ribbons (O'Connor 1967, 1972; Eby 1977). Observations of molar-tooth samples suggest to us that ribbons formed as open voids through which gas and water flowed before being filled by fine, equant sparry calcite crystals. We tested the gas hypothesis by experimenting with mixtures of plaster of Paris, clay, water, sugar, and yeast in glass aquaria. Yeast metabolized the sugar, producing CO 2 gas that first formed bubbles in the mud slurry, nearly identical in shape to Belt "molar-tooth" blobs. The bubbles rose to the surface and pulled water behind them, stiffening the mud. We later sealed off the mud surface with plaster. The confined gas within the stiffened mud produced vertical and horizontal expansion cracks that were nearly identical in form to vertical and horizontal "molar-tooth" ribbon shapes. Finally, we precipitated fine equant crystals of calcium carbonate, identical to "molar-tooth" calcite, by mixing solutions of Na 2 CO 3 and CaCl 2 . The close replication of "molar-tooth" blobs and ribbons by biogenic gas convincingly explains the origin of the blob and ribbon forms. This explanation satisfies all the constraints of the data. We propose that some ribbons in the Chamberlain and Helena formations formed as gas cracks in water-saturated mud within a meter of the depositional surface, and were partially filled with fine calcite spar before compaction. Other cracks in the mud were first compacted and later filled by calcite. Still other "molar-tooth" structures probably formed deeper in the sediment column. Patterns of "molar-tooth" structure are commonly repeated within sedimentary cycles up to tens of meters thick, indicating that they may have formed during periods of lowstand between depositional cycles. Biogenic gas generated in Belt sediment probably included H 2 S, as evidenced by microcrystalline pyrite in "molar-tooth" calcite, CO 2 , and methane. Rapid calcite precipitation in the gas voids is probably necessary for "molar-tooth" preservation and may be a secular feature of the Middle and Late Proterozoic.