Some years ago Professor Austin F. Rogers of Stanford University described a new locality in California for the mercury oxy-chloride, eglestonite,1 which heretofore had only been reported from the original locality find in the upper levels of the cinnabar deposits at Terlingua, Brewster County, Texas.2 The California locality is situated in the foothills of the Coast Range about two miles west of Redwood City, San Mateo County, where the various mercury minerals were deposited along the joints and fissures of a pale brown siliceous rock which is a replacement of serpentine. Known as “quicksilver rock,” this siliceous type of vein material is fairly common in the various mercury deposits of the Coast Range. Recently the author visited the San Mateo prospect in search of specimen material and while working on a new section of the vein about a foot below the surface, noticed groups of small, red, acicular crystals floating on drops of native mercury and in some instances lining small vugs of dolomite crystals. Examined under the microscope, these orange-red crystals proved to be montroydite, an orthorhombic mercuric oxide, which heretofore has been reported only from the original locality at Terlingua, Texas, where it was described by A. J. Moses.3 The name montroydite was given this mineral in honor of Mr. Montroyd Sharp, one of the mine owners at Terlingua. Associated mercury minerals found in the San Mateo County deposit are eglestonite in cubic, dodecahedral, and acicular malformed crystals and also as crusts coating dolomite; calomel in masses of small colorless euhedral crystals; native mercury in drops along seams and vugs in the vein; and cinnabar in subhedral crystals and crusts. In some of the vugs there is a greenish yellow powder which suggests the mercury oxychloride, terlinguaite, but not enough of the mineral is present to make conclusive tests.

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