Widely scattered bodies of Tertiary intrusive porphyry crop out in the northern Black Hills, in greatest abundance in a circular area of about 10-mile radius with Lead and the Homestake Mine at the center. Five rock types are represented: rhyolite; quartz monzonite; monzonite; grorudite; and phonolite. A swarm of rhyolite dikes in the Homestake Mine comprise rocks of two distinct compositional types: so-called low potash rocks with about equal parts of orthoclase, albite, and quartz; and high potash rocks which consist of orthoclase and quartz. There are no ferro-magnesian minerals in either type of rock.
While only a few other occurrences of rocks of the high potash type have been described, it does not seem possible to dismiss this rock type from a genetic classification on the basis of inaccurate description. Three possible modes of origin for similar rocks have been proposed: selective assimilation of potash-rich wall rocks; hydrothermal introduction of potash; or, most probably, normal differentiation with strong reaction, followed by hindered reaction (Bowen, 1928).