484 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



2. The major feature of the Western Cape region (28° and 114°) is the 

 northwesterly trending Sierra Vizcaino, bordered on the north and east by De- 

 sierto de Santa Clara. Crystalline rocks, including small areas of the Franciscan 

 formation, constitute the bedrock complex of Sierra Vizcaino the islands farther 

 northwest. Tertiary and probably some Cretaceous sediments, dipping in gen- 

 eral northeastward toward the synclinal desert and southward toward the ocean, 

 overlie this bedrock complex. Volcanism is a major feature of the southeastern 

 part of this area. 



3. The areal geology in the South-Central area is dominated by volcanic 

 rocks of Tertiary age, which obscure some of the earlier marine formations, but 

 where these formations are exposed, they usually occupy the axis of a syncline 

 and a part of the sierran area, which is anticlinal. Crystalline rocks, probably 

 elevated by faulting, are exposed on the southwest coast at Bahia de Magdalena, 

 but only small areas of these rocks, at relatively low elevations, occur along 

 the uplifted gulf coast of the peninsula. 



4. The Southern Cape region (24° and 110°) is looked upon as a distinct 

 structural block and is almost entirely granitic and metamorphic rocks, although 

 some marine Tertiary sediments occur east of the high sierra. 



5. The islands adjacent to the peninsula are composed principally of vol- 

 canic and granitic rocks. 



Metamorphic and Intrusive Rocks 



Regarding the metamorphic and intrusive rocks older than the Lower 

 and early ( ? ) Upper Cretaceous San Fernando formation, which itself in 

 places is metamorphosed, the following passages are quoted from Beal 

 (1948): 



Lindgren (1889) states that the principal mass of the peninsula at 32° N. 

 Lat. is an enormous granitic plateau with minor areas of highly metamorphosed 

 and compressed slates, the granites appearing to be a "white hornblende granite 

 similar to that of the Sierra Nevada of California." Emmons and Merrill (1894) 

 in their examination of the area adjacent to the 30th parallel found rocks of 

 the same type as those mentioned by Lindgren and to the eastward found 

 metamorphic slates which led them to remark on the similarity of structural 

 conditions and lithological character of the rocks in the two areas. According to 

 Hirschi the granitic zone of the Northern district is flanked on the gulf 

 side by old crystalline schists, of a sort not observed on the Pacific side; 

 and, in the desert sierras west of the mouth of the Colorado, great and varied 

 schist zones occur, which extend southeastward along the gulf coast almost to 

 the 28th parallel. 



The metamorphic rocks, mapped with the intrusive granitics, were observed 

 during this study to consist of gneisses, slates, schists, and other metamorphics; 

 they are exposed on both flanks of, and on, the batholith which makes up the 

 axis of the northern part of the peninsula and are known farther north on both 



sides of the batholith in San Diego, Imperial, and Riverside counties, California. 

 Lindgren (1888) states in referring to the slates on the west side of the range 

 at the latitude of Ensenada that "one cannot fail to be impressed by the enor- 

 mous extent of the granite and the small area occupied by metamorphic rocks. 

 It seems evident that the slates are of but litde depth and everywhere are rest- 

 ing, as detached fragments, on the granite." 



. . . Woodford and Harriss (1938), in a careful study of the granitic and 

 associated metamorphic rocks adjacent to 31° N. Lat., state that the crystalline 

 rocks consist of stocks and batholithic masses of quartz diorite. They consider 

 that the plutonic rocks in northwestern Baja California "are typically quartz 

 diorite, as contrasted to the granodiorite or quartz monzonite, which is the com- 

 monest rock of the Sierra Nevada." . . . 



... In parts of the Western Cape region the granitic rocks are greenish-gray 

 diorite and pink granite, occasionally cut by large intrusions of serpentine. 

 According to Hanna (1927), chert, presumably of Franciscan age, occurs on 

 Isla San Roque, Isla Asuncion, Islas San Benitos, and possibly on Isla San Ge- 

 ronimo, far to the north just below the 30th parallel; he also reports (1925, 

 p. 268) "Franciscan cherts, sandstones, and conglomerate" on Isla Cedros. At 

 Punta San Hipolito (on the south coast of Western Cape region) are quartzites, 

 cherts, cherty limestones, and igneous rocks, which were mapped as the San 

 Fernando formation but may be older. In this area, as well as near Punta 

 Asuncion, the granites and metamorphic rocks are intruded by dikes of serpen- 

 tine, but they have failed to alter the near-by Eocene sandstones which usually 

 dip toward the crystalline rocks, indicating that the serpentine dikes are older 

 than the Eocene and that the Eocene has been brought to its present position 

 with reference to the crystalline rocks by faulting. At Punta San Pablo, about 

 25 kilometers northwest of Punta Asuncion (Scammon Lagoon quadrangle), 

 Hirschi and De Quevain (1933) state that the greenish-black rocks of the high 

 coast line are probably of amphibolite and gabbro (?) broadly intruded by 

 pegmatites, and that on the south end of the "intensively folded Paleozoic 

 range" of Isla Cedros they observed strongly altered, glaucophane-bearing dia- 

 base porphyritic dike rocks. 



The same authors refer to a great peridotite intrusion at Cabo San Lazaro 

 and Punta Entrada (Magdalena Bay quadrangle), which is shattered, pene- 

 trated by east-west dioritic or dioritic porphyry dikes, and usually wholly altered 

 to serpentine. They refer to andesitic and basaltic rocks of Tertiary age, which 

 overlie the basement complex exposed along Bahia de Magdelena. Lindgren 

 (1889) states that Isla Santa Margarita is composed principally of crystalline 

 schists, with some chloritic slaty rocks and talc and serpentine. The numerous 

 reported and observed occurrences of rocks of Franciscan character constitute 

 good reason to believe that the Franciscan formation of California extends 

 southward as far as the Western Cape region and perhaps to Bahia de Magde- 

 lena. 



The metamorphic rocks of the Nevadan complex of Baja California can 

 be judged better by comparison with their northward continuations in 



