Tuff

Tuff, a general term for all fragmentary volcanic rocks, as distinguished from lava, which includes all the compact ones. Tuffs, therefore; range from the most fine-grained volcanic ashes to the coarsest agglomerate. Tuffs may have consolidated under water, or on land, or not at _all, and are generally stratified, and not uncommonly fossiliferous. Felsite-tuff and trachyte-tuff are so named from the rocks by the disintegration of which they have been formed. Palagonite-tuff contains the basalt glass named from Palagonia in Sicily; peperino is a dark-brown earthy tuff in the Alban Hills; trass, a light-coloured variety, quarried for hydraulic cement-making in the Eifel; and schalstein, sometimes at least a diabase-tuff impregnated with carbonate of lime.