The Pomona Basalt, an extensive tholeiitic lava flow of the Columbia River Basalt, overlies an airfall vitric tuff in south-central Washington. In many places, this tuff was welded at the contact from the heat and pressure of the overlying basalt. Textures and minerals (mainly Na-sanidine and cristobalite) formed in the tuff during and after welding are identical to those of welded ash flow tuffs. Heat conduction from the lava could have accomplished the welding, as shown by calculations, although water vapors greatly aided in heat transfer. Welding was completed prior to deuteric crystallization of the Pomona Basalt. In other places, where the tuff was probably thicker, the Pomona flow burrowed into the tuff, fragmented it, and formed spectacular breccias (pépérites). The pépérites occur sporadically between outcrops of the un-brecciated Pomona in an area of about 5400 km2 (2100 miles2).