The occurrence of grooves on the surface of volcanic glass shards in marine and nonmarine environments suggests the existence of microorganisms that can dissolve silica substrates. The etchings occur within a 30-cm-thick marine turbidite bed in the Miocene Obispo Tuff at Shell Beach, central California, and also in a tuff layer (Deep Creek Tuff) in the Miocene John Day Formation of eastern Oregon. The etchings occur in three different patterns with two subtypes, depending on the formation in which they occur. Type A is an anastomosing pattern of semicircular grooves covering the entire grain. Subtypes of A are distinguished by the presence (Obispo Tuff, Type A-1) or lack (John Day, Type A-2) of annulations. Type B has some sinuous, branching grooves covering only part of the grain. Subtypes are the same as those of Type A. Type C, occurring only on Obispo Tuff grains, is a series of straight, nonbranching grooves on one side of platy shards. The grooves range in size from 4–20 µm in the Obispo Formation to 15–18 µm in the John Day Formation. The grooves are strikingly similar to grooves occurring on carbonate grains.