Experimental results of laboratory measurements at ultrasonic frequencies on dry opaline rocks are documented from the Monterey Formation, a siliceous unit in the San Joaquin basin. In these samples the silica is primarily biogenic opal-A in origin, but progressively alters to opal-CT and then quartz with depth. The diagenetic transitions are accompanied by a marked porosity reduction. Compressional and shear wave velocities were measured on core plugs at effective reservoir pressure. Experimental results on samples from the opal-A/opal-CT transition and the opal-CT/quartz transition show very little pressure-dependence of elastic-wave velocities. When their elastic moduli are normalized by those of the solid phase, the moduli-porosity trend is very close to that of chalk and, in the lower-porosity domain, of some clean sandstones.¿ 1997 American Geophysical Union