Obsidians occur in many parts of the world along with rhyolites and pumice.
Included in this group are some rocks which are more properly to be regarded as vitreous forms of trachyte than as glassy rhyolites (Iceland), but except by chemical analyses they cannot be separated.
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 '
Rhyolites were erupted locally near Tardree, Ballymena and Glenarm.
The whole of this consists of tuffs and lavas, andesites prevailing in the west and rhyolites and dacites in the east.
The Devonian rocks at Yalwal are sharply folded and are associated with a series of rhyolites and basic lavas.
We may note the pre-Cambrian lavas and tuffs of the Wrekin district in Shropshire and the somewhat later volcanic rocks of Charnwood; the porphyrites, andesites, tuffs and rhyolites of the Borrowdale volcanic centre, erupted in the Ordovician period, and the Silurian granites of the same region.