Recognition of peperite is important in establishing broad contemporaneity of magmatism and sedimentation. At an occurrence of peperite discovered recently among late Paleozoic island-arc deposits in the northern Sierra Nevada, it is possible to reconstruct the peperite-forming process largely from field observations alone. The generally concordant, upper margin of a basalt intrusion adjoins thinly laminated basaltic tuff in the Taylor Formation that was water saturated and unlithified at the time of intrusion. The water-saturated ash initially was heated by the intrusion at approximately constant, essentially hydrostatic pressure. Concurrently, the top of the intrusion was quenched and divided by shrinkage fractures. Pressure momentarily approached zero as the shrinkage fractures opened abruptly, causing vaporization of the pore water and substantial expansion and fluidizing flow of the resulting steam. The unlithified ash was swept into the shrinkage fractures, and, where the downward-propagating fractures intersected one another, basalt fragments were detached to form in situ peperite. Associated mesoscopic folds in thinly laminated tuff are shown to be a consequence of the peperite-forming process; they need not be attributed to forcible intrusion of the basaltic magma.