Acahay

Acahay forms an annular ridge rising 300 m above the surrounding plain and has a deep central basin. Milton and Eckel (1959) distinguish on air photographs numerous small cones to the east and northeast, as well as dykes. 'Gabbros', 'syenites' and 'basic diorites' are reported, as well as a 'basalt' described as containing "pale-green pyroxene" in a groundmass of sodic plagioclase, with a few iddingsitized brown olivine phenocrysts (Milton and Eckel, 1959). Putzer and Boom (1962) present a mode of an essexite as plagioclase + orthoclase 37.1%; nepheline 5.7%, pyroxene 41.1%, biotite 5.1%, and olivine + ore + zircon + apatite 10%; a chemical analysis is also given. A large phonolite dyke trends northwest-southeast (J.H. Palmieri, personal communication, 1981).