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.
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