Sierra San Pedro Mártir of northern Baja California is an elongated plateau—a slightly dissected fault block—which trends north-northwest along the axis of the peninsula. In a central east-west strip across the Sierra and its western foothills at about 31° north latitude the rocks include (1) an extensive metamorphic series of varied slaty and schistose rocks, striking northwest, and occurring chiefly in the foothills, (2) gneissic migmatites; occurring chiefly on the plateau.; and (3) stocks and batholiths of quartz diorite and granodiorite which are of uncertain, perhaps Upper Cretaceous age.

The present report includes a reconnaissance geologic map of the previously almost unexplored area, and descriptions of the rock types found. Further work in this and neighboring areas is desirable, especially to clarify the structure and geomorphology, and to ascertain more definitely the ages of the various metamorphic and plutonio rocks.

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