Mount Diablo is an isolated peak of the Coast ranges of California, lying about 27 miles east-by-north of San Francisco. Although less than 4,000 feet high, it forms a prominent feature in the landscape, rising as it does from sea level, and is visible from all the main lines of railway leading to San Francisco. Owing to its isolation, the view from the summit is unusually fine, and it is frequently ascended by excursion parties.
Being easily accessible, it was selected by Professor Whitney as a field for detailed geological work when he first undertook the study of the Coast ranges. He there first obtained evidences of the Cretaceous age of the bulk of the metamorphic rocks of the range, having observed there the unaltered shales containing Aucella mosquensis, Von Buch, passing into silicified shales, or phthanites, about the head-waters of Bagley creek.*Aucella mosquensis† is a . . .