Among the Tertiary volcanic rocks those of acid types (granites, granulites) were the first to appear and are developed latitudinally; rocks of intermediate type (dacites, andesites) characterize the Miocene and early Pliocene periods; while the basic rocks (ophites, elaeolite syenites and basalts) attained their maximum in later Pliocene and Quaternary times.

The whole of this consists of tuffs and lavas, andesites prevailing in the west and rhyolites and dacites in the east.

The rocks are liparites, dacites, hornblende and pyroxene andesites.