Representative collections of the richer silver ores from the famous Mexican districts of Pachuca and Real del Monte are now available mainly from Museum sources. Microscopic study of excellent collections from the U. S. National Museum reveals an unusual degree of uniformity in the character of the mineralization throughout the area. The primary mineralization represented a single episode characterized by a comparatively simple mineralogy. Pyrite, quartz, and sphalerite were the early hypogene minerals followed by hypogene galena, chalcopyrite, argentite and rarely polybasite and stephanite. The principal primary (hypogene) source of silver is unquestionably argentite. Native silver appears always to be secondary (supergene). Some argentite is also supergene.

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