Baja California is about the same length (1300 km.) as California and has a little more than a third its area. Essentially it is a westward-tilted fault block, with its mountainous northern half consisting mainly of crystalline rocks, which rise to elevations of more than 10,000 feet. Most of its southern half is a great area of volcanic flows and detrital rocks, overlying crystalline rocks and unaltered Tertiary marine sediments of pre-Upper Miocene age.

The formations exposed are the partly metamorphosed Lower and early (?) Upper Cretaceous San Fernando formation, unconformably overlain by the unaltered Upper Cretaceous Rosario formation; the...