The Tuscan Formation consists of dominant tuff breccia and lapilli tuff, and minor lava flows, flow breccias, and tuff; volcanic conglomerate, sandstone, and siltstone are important constituents in its western portion near and in the subsurface of the Sacramento Valley. Chemical analyses from the major source areas suggest that breccia clasts of the Tuscan Formation are andesite and basaltic andesite. The formation has a maximum exposed thickness of 1700 feet and once covered about 2000 square miles.

Silicic ash-flow tuffs, included in the Tuscan Formation, previously have been correlated with the Nomlaki Tuff Member, but as several silicic tuff units are present, such correlations are not always justified.

The late Pliocene age of the Tuscan Formation is supported by nonmarine diatoms. Radiometric dates and the almost total absence of paleosols in the Tuscan Formation suggest that its lahars were emplaced relatively rapidly, probably in less than a million years.

Maps of the distribution and thickness of the Tuscan Formation are presented, from which it is inferred that laharic debris originally amounted to 300 cubic miles.

Principal source areas of the Tuscan lahars include two eroded composite volcanoes south of Lassen Peak and two lesser source areas of indefinite form northwest of Lassen Peak. Small groups of tuff-breccia dikes west of the volcanoes apparently contributed only slight amounts of debris.

Mount Yana, the chief source of the Tuscan and the southernmost volcano of the Cascade Range, was constructed chiefly of flows of pyroxene andesite and interbedded thick laharic units. A dike swarm that included many tuff-breccia dikes invaded the central part of the volcano. After volcanism ceased, slight fault movement disrupted the central part of the volcano and facilitated erosion, which then excavated a large central depression. Structural control was exerted by a major east-west lineament that probably marks the structural northern limit of the Sierra Nevada. Indirect evidence suggests vertical movement of about 2500 feet in Pliocene time.

Water needed for the mobility of the Tuscan lahars probably did not come from extensive fields of snow or ice. Heavy rainfall is a possible source of water, but estimates of available magmatic and meteoric water show that ample water is available from these sources.

Significant proportions of the lahars formed by near-surface autobrecciation in dikes and central conduits at temperatures less than 800° C, and probably in the range 340° to 280° C.

The mechanism of brecciation proposed by Curtis (1954) probably was effective at Mount Yana.