During the course of an investigation of cinnabar ores, it was found that some specimens show the peculiar tendency to turn surficially black, relatively rapidly, on exposure to sunlight. Insofar as the writer is aware, this rapid darkening is shown by cinnabar from only the following four localities: (1) the Opalite mine and associated deposits located in southeastern Oregon, a few miles north of the town of McDermitt on the northwestern Nevada-Oregon border; (2) the Goldbanks deposit located about thirty five miles south of Winnemucca, Nevada; (3) some of the cinnabar in siliceous sinter at Steamboat Springs, Nevada; and (4) ore from the B and B mine in Esmeralda County, Nevada. The darkening of cinnabar at the third locality is known to the writer only through conversation with Professor V. P. Gianella and that at the fourth locality, through information received from Dr. R. W. Webb. At all four of these localities the cinnabar occurs disseminated through hydrothermal silica formed at or near the surface. At least in the Goldbanks deposit, the cinnabar and silica have been deposited syngenetically and the admixture is so intimate that it is impossible to effect a complete separation of cinnabar from silica. Ransomel has stated that all cinnabar darkens on exposure to sunlight. Upon exposure for a long period, this may be true, since it likewise has been noted that cinnabar vermilion used in painting darkens alter a number of years.2 However, within a period of a few months, or a somewhat longer period, this darkening is not characteristic of all cinnabar since the writer has seen a number of museum specimens and open pit exposures which are still bright red after exposure to sunlight for several years. A specimen of cinnabar from the Aurora mine in San Bbnito County, California, as well as cinnabar concentrated from a specimen collected by Ransome in the Mazatzal Mountains of Arizona were exposed to sunlight for six months without any change being noted.