Tuff - Basaltic Tuff

Basaltic Tuff

Basaltic tuffs are also of wide spread occurrence both in districts where volcanoes are now active and in lands where eruptions have long since ended. They are found in Skye, Mull, Antrim and other places, where there are Paleogene volcanic rocks; in Scotland, Derbyshire and Ireland among the carboniferous strata; and among the still older rocks of the Lake District, southern uplands of Scotland and Wales. They are black, dark green or red in colour; vary greatly in coarseness, some being full of round spongy bombs a foot or more in diameter, and, being often submarine, may contain shale, sandstone, grit and other sedimentary material, and are occasionally fossiliferous. Recent basaltic tuffs are found in Iceland, the Faroe Islands, Jan Mayen, Sicily, Sandwich Islands, Samoa, etc. When weathered they are filled with calcite, chlorite, serpentine and, especially where the lavas contain nepheline or leucite, are often rich in zeolites, such as analcite, prehnite, natrolite, scolecite, chabazite, heulandite, etc.