Distinctively blue upper Tertiary sandstones, composed almost entirely of andesitic detritus derived from upper Tertiary andesite breccias of the northern Sierra Nevada, are widespread in central California. These volcanic arenites are well sorted and loosely cemented by a thin coating of authigenic clay mineral encasing each detrital grain. The clay has grown in laths or fibers oriented perpendicular to the surfaces of the clastic grains. The blue color is due to the effect of this translucent coating on light reflected from the dominant dark-colored grains beneath it. Optical, X-ray, differential thermal, and chemical data indicate that the mineral is a montmorillonoid intermediate between beidellite and nontronite, a composition not commonly recorded in published analyses. The clay mineral crystallized from pore solutions permeating the sandstone, the material probably being mostly derived by solution of the groundmass of the andesite rock fragments, which make up half or more of the sandstone.