The coexisting pyroxenes and olivines from nine British and Icelandic Tertiary porphyritic acid glasses have been separated and analysed chemically. Eight new analyses of augite and ferroaugites, one of orthopyroxene, and five of iron-rich olivines are presented, together with their optical properties. The trend of crystallization of the ferroaugites is discussed. New optical determinative curves for these ferroaugites and the iron-rich olivines are presented. The relationship of the pyroxenes and olivines to their analysed residual glasses is considered.