INTRODUCTION
Silica-rich rocks are scattered throughout the Berkeley Hills. Volcanic rocks known as the Leona and Northbrae Rhyolites were first described by Andrew Lawson (1914). Lawson considered the Northbrae as a flow-banded subset of the generally featureless Leona and considered them to be of Plio-Pleistocene age. Since 1914, the Northbrae and Leona Rhyolites have been subject to various interpretations. Knox (1973) and Dibblee (1980) mapped them as Tertiary flows. The geologic map of the San Francisco-San Jose Quadrangle (California Division of Minds and Geology, 1991) identified these rocks as "rhyolite of uncertain age" and tentatively grouped them as part of the Jurassic Coast Range Ophiolite (CRO). The United States Geological Survey (Open File Reports 94-662 and 96-662) characterized the Leona and Northbrae as volcanic rock (keratophyre and quartz keratophyre) overlying the CRO. Although now mapped as one unit, the widespread Leona and the areally restricted Northbrae are distinguished from each other by their petrology, geochemistry, and morphology. Recent dating indicates that the Northbrae is a late Miocene (11.5 Ma) flow. With the exception of Cragmont Rock, which was mapped as Franciscan chert (Dibblee, 1980), other silica-rich rocks in the Berkeley Hills, such as at Hinkel and Remillard Parks have not heretofore been separately mapped or described.
