| Only andesitic tuffs have phenocrysts, amygdaloidal structures, and slight flow structures. |
| They include lavas and volcaniclastic rocks of basaltic and andesitic composition together with cogenetic gabbros, plagiogranites and granites. |
| The tuffs and lava were derived from a nearby andesitic volcanic centre. |
| The injection of a mafic magma into a more silicic magma chamber is an important trigger mechanism in moderate-sized andesitic, dacitic and rhyolitic eruptions. |
| In the andesitic rocks zircon is less abundant than in the porphyroids. |
| The Longcroft Tuffs, a poorly exposed succession at least 140 m thick, are dominated by lapilli tuffs of andesitic composition. |
