An occurrence of dumortierite, heretofore undescribed so far as known, is in the vicinity of the Cajalco tin mine, property of the American Tin Corporation, approximately 13 miles southwest of Riverside, California. Apparently the mineral is rather widespread in the region, as suggested by a specimen collected by Charles Anderson from a point four miles northeast of Corona.1 The dumortierite occurs in a granodiorite country rock whose chief constituents in apparent order of abundance are: orthoclase, oligoclase, quartz and biotite. The rock is hypidio-morphic granular, coarse textured and rich in quartz. Accessory minerals are titanite, magnetite and apatite. The rock generally carries more or less tourmaline, especially near each of the numerous well-defined tin-bearing tourmaline-quartz veins on the property.