The National Park Service, which actually has jurisdiction over Devils Tower, flatly states it "is formed of a rare igneous rock, phonolite porphyry" and is a geological marvel, not a botanical one. " ...

The occurrence of porphyritic daci-andesite intrusive rocks and hydrothermal brecciation exhibit strong quartz-sericite (phyllic) alteration overprinting now relict K-feldspar alteration that host ...