Structural evidence, particularly in the deeper mine‐workings, indicates that the volcano, which is of Tertiary age, was developed by explosive eruptions at a number of points along intersecting fissure‐systems that had been formed in pre‐Cambrian granite by east‐west compression, probably during the Laramide revolution. The volcanic breccia, which consists principally of phonolitic material, appears to fill several elongate or elliptical funnel‐shaped openings that coalesce upward and taper downward into roots. Local adjustments that followed the principal explosive eruptions developed fissure‐systems in the breccia parallel and transverse to the older fissure‐systems and permitted the intrusion of dikes and irregular masses of latite‐phonolite and syenite within the breccia mass. Mild regional readjustment, caused by compression in a nearly north‐south direction, ensued and reopened these fissure‐systems within and around the breccia mass. It was followed by the relatively widespread intrusion of dikes and irregular masses of phonolite. Local readjustment then recurred repeatedly, mainly along the master fissure‐systems, and was followed by three successive intrusions of alkaline basaltic dikes and finally by the formation of mineral veins.
