Welded rhyolite tuff, of Pleistocene age and having an areal extent of 400 square miles and an average thickness of 500 feet, occurs between Bishop and Mono Lake in eastern California. In its upper portions, pumice lapilli are imbedded in a porous vitric-crystal matrix. In sections several hundred feet thick, there is a textural and structural gradation from top to bottom, wherein, at the base, vitric constituents are compressed, distorted, and aligned in the horizontal plane, and the structure becomes very compact. This is the result of the compression of the basal parts of a heated pyroclastic mass by the weight of overlying material. In some thick sections, the glassy constituents have crystallized to fibrous aggregates of tridymite and potash feldspar. In the field the tuff is characterized by an absence of bedding, by columnar jointing, and by a surface which is remarkably even despite the irregularities of the buried topography.

The vents are concealed by the tuff but are probably scattered through the region along faults which have been active both before and since the eruptions. The type of eruption represented is similar to the nuées ardentes and comparable in detail to the “Katmaian sand-flow” of Fenner, a type which is being widely recognized in recent work.

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