The older Precambrian rocks of Arizona include the Vishnu, Yavapai, and Pinal schists, all overlain unconformably by nonmetamorphosed younger Precambrian rocks. The older Precambrian schists, unnamed gneisses, and associated granitic masses crop out in many of the mountain ranges southwest of the Colorado plateau. The stratigraphy and structure of the schists can be unraveled to some extent by detailed mapping, and work now in progress by the U. S. Geological Survey in the Bagdad, Prescott-Jerome, and Little Dragoon areas, is revealing folded structures trending generally northwest to northeast. Earlier work by Wilson in the Mazatzal Mountains revealed southeastward-dipping, low-angle thrust faults of older Precambrian age. In the Bagdad and Prescott-Jerome areas, a variety of igneous rocks, including rhyolite, alaskite porphyry, diorite, and gabbro, were intruded into the schists prior to the widespread invasion of granite. Only one period of orogeny followed by the intrusion of granitic rocks can be recognized in each area studied to date. The degree of metamorphism is not uniform and some of the pre-intrusive rocks are nonfoliated, whereas others are highly schistose. Some of the intrusive rocks, including granite, show the effect of dynamic metamorphism. The grade of metamorphism is low to intermediate, except near the large masses of granite where coarse-grained sillimanite-bearing schists are found.
