Thin-section and washed-residue examination of a well-indurated pebble conglomerate from the base of the Butano (?) Sandstone of Dibblee and about 15 feet stratigraphically above depositional contact of the conglomerate with a sill-like serpentine intrusion on Jasper Ridge, San Mateo County, California, reveals an extraordinary assemblage of plant and animal microfossils that date the stratum as the oldest marine Tertiary unit thus far recognized in the structurally complex southwestern part of the Palo Alto quadrangle.

Conspicuous in the microfauna which was isolated from the calcareous matrix are Alabamina wilcoxensis Toulmin of Mallory and Discorbis baintoni Mallory, both early Tertiary index foraminifers in California. These, together with several other rotaliines and a few miliolids, cibicidids, anomalinids, textulariids, and globigerinids, suggest that the Jasper Ridge conglomerate was laid down in neritic waters that had a limited access to the open sea in late Paleocene (Bulitian) or early Eocene (Penutian) time, according to Mallory's tabulation of these protozoans in Paleogene strata of the California Coast Ranges.

The conglomerate crops out about midway between the San Andreas fault zone and the distorted sedimentary section exposed in the trench for the Stanford linear accelerator. Therefore, the early Tertiary segment is in an area that has been profoundly affected by numerous diastrophic events. The lithologic character itself furnishes significant paleogeologic and paleogeographic data, because the pebbles appear to have been derived mainly from a Franciscan terrane. The dominant pebble type is greenstone, with relic basaltic and andesitic textures, and characterized by albite, chlorite, pumpellyite, and other low-grade metamorphic minerals. Pebbles in small amounts include chert, graywacke, limestone, felsite, quartzite, semi-schist, and metagabbro. About 95 per cent of the pebbles are wel -sorted basalt and 3 per cent red radiolarian chert.

It could not be ascertained whether the serpentine layer was intruded during Cretaceous time into the Franciscan on Jasper Ridge as Dibblee reported in 1966. However, the fact that the serpentine was exposed at least during the early Tertiary is confirmed by its juxtaposition with the fossiliferous conglomerate. Correlation with the large, plug-like serpentine mass described in 1951 by Thomas from exposures 2-3 miles toward the northwest is postulated, although the Redwood City area ultrabasic body was considered by Thomas to be a cold re-intrusion into Eocene strata, emplacement having occurred between late Eocene and early Miocene.

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