The old copper mines of Carroll County, Maryland, are known to mineralogists as the type localities for two rare cobalt minerals, remingtonite and carrollite. The ores have, however, received little critical mineralogical examination in recent years. While on a visit to several of the mines in company with Drs. Schaller and Foshag the writer found a specimen of a deep blue mineral at the Mineral Hill mine and the same mineral was found at the Patapsco and Springfield mines by Dr. Foshag. This mineral had much the appearance of lazulite but upon optical examination it was found to be isotropic and analysis proves it to be the zinc spinel, gahnite, notable as owing its beautiful blue color to cobalt which is present in definite amount. This analysis is interesting as confirming the work of Wherry who, by microspectroscopic examination, found that distinctly blue spinels gave the absorption spectrum of cobalt.2 Trial of a thick section of this Mineral Hill occurrence showed only a faint, hazy band, but it occupied the characteristic position in the spectrum, wave length 550.