Laumontite of hydrothermal origin occurs in veins and as a replacement of the anorthite component of plagioclase in granitic and gneissic basement and overlying arkosic sandstone and conglomerate of the San Francisquito(?), Vaqueros and Cajon formations exposed in Cajon Pass north of the San Andreas fault. Accompanying alteration includes exsolution of albite from plagioclase, conversion of orthoclase to microcline and perthite, and oxidation of iron in biotite and magnetite to form a disseminated hematite stain. These changes are pervasive within 1 km of the fault but diminish to no alteration at 4 km. Granitic rocks close to the fault are crushed to highly fractured and contain a mosaic of laumontite veins. Veins occur along joints and minor faults in sedimentary rocks. Most veins are nearly vertical and trend northwest at a more northerly angle than the San Andreas. This mineralization is attributed to deep circulation of ground water in fractured and frictionally heated rocks along the San Andreas fault. ¿ American Geophysical Union 1988