Two ash-flow sheets of markedly different age and petrography are exposed in the Castillo de Callalli. The lower, phenocryst-rich ash-flow sheet of low-silica rhyolite has a K-Ar age of 20.7 -+ 0.6 Ma. The overlying ash-flow sheet is composed of sanidine-bearing high-silica rhyolite. An 40Ar/39Ar age of 4.72 +- 0.02 Ma and petrographic characteristics show that the unit is almost certainly the lower ash-flow sheet of the Cailloma Tuff, which in adjacent quadrangles has been mapped variously as Fm. Sencca, Fm. Garza, Fm. Yauri, and Ignimbrita Acopata.

El Cañón del Colea, es parte del valle homónimo perteneciente a vertiente pacífica, ubicado políticamente dentro las provincias de Castilla y Cailloma, Departamento de Arequipa. Comprendido entre las localidades de Pinchollo (2,800 m.s.n.m.) hasta el sector de Andamayo (920 m.s.n.m.), donde el río tiene un recorrido subrectilineo de áproximadamente 93 Km. y 70 km en línea recta.

High-silica rhyolite is present in various parts of southern Perú. Rhyolite from two localites, the Cailloma caldera and the newly recognized Río Chalhuanca dome field, both in Arequipa Department, belong to the group of "topaz" rhyolites well known in the eastern and central parts ofthe Great Basin and New Mexico, Western U.S., and in many localities in Mexico ( e.g., Christian sen et al., Geol. S oc. Am. Spec. Paper 205). The Peruvian occurrences may be the first topaz rhyolites recognized in South America.

Versión traducida al español del original en idioma inglés del año 2003, que se incluye en esta publicación.