The principal rocks are andesites, but trachytes and basalts are also common.
The rocks are mainly basalts and andesites, together with trachytes and phonolites, and some of the basaltic flows are of enormous extent.
Trachytes, rhyolites, andesites and basalts occur, and a definite order of succession has been made out in several areas; but this order is not the same throughout the chain.
The building up of these domes of lavas of intermediate chemical type was followed by the eruption of sheets of andesites and rhyolites in the Thames English Miles so Ioo 200 Cretaceous 'a '
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.