Igneous rocks of the Highwood Mountains, Montana: Part II. The Extrusive Rocks

Early eruptions of quartz latites, rather rich in potash, built up a volcanic mountain over 30 miles across on an irregular surface of late Cretaceous sediments. Erosion then removed much of the quartz latite. Renewed volcanism formed a volcano made up of basaltic rocks (mafic phonolite) that covered the quartz latites. The phonolites vary in texture and in the kind and proportion of the felsic minerals present. All contain abundant phenocrysts of augite and some of olivine; some have phenocrysts of leucite, others analcime, and many have pseudoleucite. Some have phenocrysts of biotite and barium sanidine. The groundmasses contain about the same minerals as the phenocrysts with more felsic minerals and more biotite in the coarser-grained groundmasses.