Tuff: "indurated pyroclastic rocks of grain generally finer than 4 mm, that is the indurated equivalent of volcanic ash or dust" Wentworth and Williams (1932)

Pyroclastic: an adjective commonly applied to rocks produced by explosive or aerial ejection of material from a volcanic vent. Wentworth and Williams (1932)

Tuff, welded: tuffs in which the individual particles remained plastic enough to become partly or wholly welded.

Wentworth, C.K. and Williams, Howell, 1932, The classification and terminology of pyroclastic rocks, Natl. Research Council Bull, 89, p. 19-53.

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