The color variations are swirled like marble-cake in many places. However, the marble is fractured and not very strong, and so is not suitable for building stone. Minerals Many interesting minerals can be found at Fort Irwin. Most are in igneous and metamorphic rocks. Igneous rocks formed from melted rock, or magma, either deep in the earth as granite, or on the surface as volcanic rocks such as rhyolite and basalt.
Metamorphic rocks form at high pressure and temperature by causing new minerals to grow.
Amphibolite consists almost entirely of inch-wide black hornblende crystals, with white plagioclase feldspar filling between the crystals and a few small shiny metallic crystals of chalcopyrite scattered about. Unusual schist south of the Tiefort Mountains in one location has unusually abundant sillimanite , and has large black crystals of magnetite . Tiny red garnets can be found in granite in hills near Goldstone. This granite also contains feldspars , quartz , and muscovite . Museum photographs of minerals Archeology Several sites with relics of human occupation are known at Fort Irwin.