The Hermosillo region of Sonora, Mexico is host to a number of mesas formed of peralkaline rhyolite. Outcrops are scattered over a 25 x 40 km area and are composed of a single unit known as the Hermosillo ignimbrite, with layers oriented horizontal to gently dipping west. The unit is best observed at the mesas Cerro Colorado and Cerro Las Cuevitas, with the stratigraphy of the latter being studied in depth. The unit is 10-50 m thick composed of a black basal vitrophyre, grading to a welded grey-purple eutaxitic ignimbrite. The ignimbrite is porphyritic with phenocrysts of sodic alkali feldspar, lithic-poor, and lacking hydrous mineral phases. Analysed samples contain normative aegirine and an agpaitic index of 1.01-1.12, where basalts concordant with the ignimbrite are found to be alkaline. This volcanism is thought to occur as part of continental extension prior to the formation of the Gulf of California. Geochemical data including major and trace element and mineral chemical can be found in Vidal-Solano et al. (2005).