San Francisco Mountain

The San Francisco volcanic field comprises numerous cones and extensive flows of basalt, latite, dacite and rhyolite, San Francisco Mountain being the largest edifice and lying towards the centre of the field. Small volumes of comendite lava are found on San Francisco Mountain, in the saddle between Agassiz and Fremont Peaks and on the northwest slope of Fremont Peak (Robinson, 1913, Plate 5). The rocks are partly spherulitic and banded, but with some non-hydrated glass. Phenocrysts of alkali feldspar and quartz are set in a glassy matrix with swarms of tiny blue and brown amphibole crystals and some zircon. One of the analyses presented by Robinson (1913, p. 109) has been repeated using the same material (Noble and Parker, 1975, Table 2).