The lamellar spacings in the sanidine cryptoperthites (bulk composition Or66.4Ab32.3An1.3) from the Bishop Tuff near Long Valley, California, have been determined using transmission electron microscopy. The lamellar spacings show a sharp maximum at about 43 m below the topographically highest sample and a weak minimum about 4 m above this. This pattern indicates that two flows compose the section. The gradational boundary between the flows is about 37 m below the highest sample. Heat-flow calculations applied to the pattern of lamellar spacings indicate that these flows were emplaced within 1–5 yr of each other. The result illustrates the power of this technique in deciphering the geologic history of igneous rocks.