The weakly metamorphosed volcaniclastic rocks of eastern Plumas and Sierra Counties, California, considered to be Jura-Trias by H. W. Turner, are almost entirely Paleozoic, ranging in age from Silurian-Devonian to middle Permian; they are correlative with the Paleozoic rocks of the Taylorsville area, as revised by McMath. The succession of formations from the base upward consists of the Shoo Fly Formation, not described in detail; the Sierra Buttes Formation, mostly quartz keratophyre; the Elwell Formation, characterized by radiolarian chert; the Taylor Formation, composed of andesite; the Peale Formation, a heterogeneous mixture of keratophyre, limestone, chert, and other rocks; the Goodhue Formation, composed of andesite, basalt, and olivine basalt; the Reeve Formation, predominantly of keratophyre; and a residue of contact-metamorphosed calcareous rocks that may be the Mesozoic Milton Formation of Turner.
Exceptions to the dominant volcaniclastic nature of this 10,900-m- (29,000-ft) thick section are a lava flow in the Elwell Formation, a thin section of massive lavas in the Taylor Formation, and minor amounts of pillow lava in the Taylor, Peale, and Goodhue Formations.
No major discontinuity has been recognized other than that between the Shoo Fly and Sierra Buttes Formations, but possible depositional gaps may occur at the base and top of the Peale Formation.
These units appear to be a partial section across an upper Paleozoic island arc that consisted of overlapping volcanoes or sheets of volcaniclastic rock, all deposited in a deep marine environment.