The Dos Cabezas Mountains lie in the northeastern most part of Arizona, in Cochise county. They area medium-sized range of some 150 square miles area, and are almost wholly surrounded by unconsolidated basin-fill material. Most of the range is composed of a number of Precambrian igneous and metamorphic rock masses. Its core is composed of a large, complex, terrain of Cretaceous intrusive volcanic breccias and magmatic aphanites. A large number of Laramide and mid-Tertiary intrusive plutons and dikes cut the range. The Precambrian rocks consist of eight granitoid plutons and three areas of phyllitic and argillitic metamorphosed sediments and volcanics. The metamorphic rocks display a primary greenschist facies dynamothermal metamorphic fabric and a later superimposed biotite-forming hornfelsic thermal metamorphic fabric. The metasediments are mostly phyllites and argillites, but contain over 10,000 feet of metaconglomerate showing marked primary cross-bedding. Many of the metamorphic units are weakly metamorphosed volcanic flows or tuff-contaminated fluvial clastic sediments. These rocks are all classified as Pinal Schist, although some may be equivalent Mazatzal Quartzite. The plutons consist of a pre-Pinal-metamorphism dacite porphyry stock, one quartz monzonite gneiss synkinematic with the Pinal dynamothermal metamorphism, and four gneissic quartz monzonite plutons which appear to post-date the Pinal metamorphism and imply a mild tectonic event at about 1450 million years ago, and two large post-kinematic quartz monzonite stocks which are of circa 1400 million years age. One of these latter stocks displays prominent rapakivi texture; this is considered to be the result of normal magmatic crystallization. The texture is caused by reaction breakdown of hornblende to form biotite among crystals floating in the magma, thereby extracting potassium from the magma and temporarily halting potash feldspar crystallization while allowing plagioclase crystallization. Rb-Sr da