Since 1924 the writer has been engaged in a study of the pre-Cretaceous crystalline rocks of southern California. As part of this program, certain areas have been selected for rather detailed geologic mapping and study of the kinds, ages, structures, origins, and modes of emplacement of the rocks. One of these “key” regions, containing an unusual assemblage of crystalline rocks, forms the subject of this paper. The work has involved the making of a geologic map of about 145 square miles, the collection and study of numerous specimens, and the examination of 115 thin sections. The study throws light upon several important problems in petrogenesis, such as remarkable variations in texture, structure, and composition of both gabbro-diorite and granite bodies, the effects of the granite intrusion upon the gabbro-diorite, the origin of an exceptional porphyry filled with large euhedral crystals of potash feldspar, the clear-cut mode of emplacement of . . .