Insoluble residues, obtained from the Lowville limestone at Bellefonte, Pa., show appreciable quantities of authigenic albite of unusual purity. The mineral occurs as twinned idiomorphic crystals, the largest of which are about one-fourth millimeter along the major diameter. Inclusions of small rhombs and granules of calcite are commonly observed in the albite crystals. The authigenic character of the albite is suggested by the unusual chemical composition, idiomorphic habit, the dearth of associated clastic materials, and the manner of distribution in the limestone. The crystals do not appear to be confined to special horizons, nor are they related genetically to fracture zones which may be considered avenues of circulation for hydrothermal solutions. The enclosing rock shows no trace of metasomatic change or heat effects of any kind, nor do the crystals appear to show replacement features, but rather they suggest formation in the calcareous muds of the sea floor during deposition and compaction of the sediment. Complete chemical analyses of the feldspar collected at random from several different parts of the Lowville rock show no potash but an unusually high soda content.