64 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. '^^vo^.Tix: 



preserved. The Jurassic epoch closed with profound folding and faulting, with the intrusion 

 of extensive masses of greenstone followed by peridotite, gabbro, granodiorite, and dikes of 

 dacite-porphyry. The concordant sill-like character of the basic intrusions is most clearly 

 seen in the map of the Galice-Kerby-Waldo region (Diller '14) where a single mass of serpentine 

 follows the strike of the country unbroken for over 40 miles. Erosion succeeded, and further 

 marine deposition followed during Lower Cretaceous (Knoxville), continuing into Middle 

 (Horsetown) and Upper Cretaceous (Chico) times. The Laramide orogeny followed, and 

 minor intrusions of pyroxenite and gabbro accompanied this folding. 



Southward the serpentine belt passes into northern California, where the serpentines 

 have been studied by Kramm ('10). The most elaborate investigations are, however, those 

 of San Francisco and La Cruz, respectively, by Lawson ('14), Branner ('09), and their asso- 

 ciates. Here, resting on an eroded surface of granite intrusive into ancient sediment, lies the 

 Jurassic ( ?) Franciscan series 4 of sandstone and conglomerates, shales, and a little limestone, 

 with abundant interlaminated radiolarian chert, intercalated vesicular basaltic rocks, and 

 irregular intrusive masses of pillow-lava (Ransome '93), generally invading sandstones and 

 especially radiolarian rocks. Some of these have a more or less alkaline nature, e. g., fourchite 

 (Ransome '94), a feature which may recall the sodic character of the spilites. The folding 

 at the close of Jurassic times was accompanied by the development of laccolites, sills, and 

 dikes of serpentine following the general strike, which greatly metamorphosed the Franciscan 

 sediments, radiolarian, and otherwise, with the formation of glaucophane-schists. "This 

 metamorphism occurred before the deposition of the Knoxville formation, and indeed before the 

 erosion-interval that followed Franciscan time, so that the intrusion of the peridotites and ellip- 

 soidal basalts must have occurred so soon after the deposition of the Franciscan strata that 

 they may be with propriety referred to Franciscan time" (Lawson '14). Indeed the occur- 

 rence of pillow-structure in the basalt suggests that they may have been intruded into uncon- 

 solidated sediments. (Cf. Lewis '14.) The schistose nature of the metamorphic rock produced 

 is, however, a very remarkable feature, though its sodic character suggests that the metamor- 

 phism may be a phase of that chemical process which produced the adinoles about the Devonian 

 diabases of Germany and S. W. England or the albitic marls by the Tertiary diabase of Italy. 

 The Lower Cretaceous (Knoxville) shales and the Upper Cretaceous (Chico) conglomerates and 

 sandstones (here concordant) were deposited on a peneplain cut from the Franciscan sediments 

 and igneous rocks. 



The region of La Cruz, immediately to the south of the San Francisco area, gives a like 

 series of events, and the unconformity between the Franciscan and Knoxville formations is 

 emphasized by the occurrence of pebbles of the Franciscan rocks in the latter (Branner '09). 

 Nutter ('01) has traced the line of serpentines for nearly a hundred miles farther to the south, 

 along the Salinas Valley. Fairbanks ('04) has described the San Luis Obispo region still farther 

 south, where again Knoxville beds occur unconformably overlying the Franciscan series, which 

 here also, though chiefly of shallow water origin, contain many thin lenticles of radiolarian 

 chert altered to glaucophane-schist where in contact with the basic intrusive rocks of the older 

 series. These are the greatly altered amygdaloidal and pillowy basaltic rocks, the pyro- 

 xenite and peridotites which were erupted at the close of Jurassic times. A later series of dia- 

 bases, serpentines, and gabbros invaded the Knoxville but not the Chico rocks, there being an 

 unconformity between the two formations here. The long period of Neogene sedimentation 

 was followed by a further folding in which was developed intrusions of olivine-dolerite and 

 augite eschenite. 



The close association between pillow-lavas, diabases, serpentines, and radiolarian rocks, 

 which is thus a marked feature of the Jurassic intrusions of the west as well as the eastern border 

 of the Pacific, suggests at once the features of alpine occurrences and the "green rocks" of 

 Lower Paleozoic age in Scotland, as Steinman ('05) pointed out. This similarity is strengthened 

 by Fairbanks's description ('96) of the rocks of Point Sal, south of the San Luis Obispo. Here 



< Reference should be made to Davis's recent descriptions ('18, '18a) ot the Franciscan Series. 



