50 p. B. KING 



the coast at a slight angle. At Point Conception, however, the 

 prominent Transverse Ranges run nearly eastward, facing the 

 Channel Islands offshore in the longitude of Santa Barbara, and 

 rimming the north side of the Los Angeles lowland farther east. 

 South of the Transverse Ranges and the Los Angeles lowland the 

 Peninsular Ranges resume a trend parallel with the coast, and form a 

 more massive set of blocks, somewhat like the Sierra Nevada. 



Dominant surface rocks of the Coast Ranges are mainly of 

 Tertiary age, primarily fine- to coarse-grained clastic sedimentary 

 strata, but with much diatomite and some volcanics. All of them, 

 even the youngest, are folded and faulted, in part heavily so, indi- 

 cating that the latest phase of the construction of the ranges was 

 late in geologic time. 



Basement Rocks. We have suggested earlier that at least the 

 northern part of the Coast Ranges, west of the Sacramento Valley, 

 was a marine realm through Jurassic and much of Cretaceous times 

 — continental shelf, continental slope, and ocean deep — which re- 

 ceived great masses of sediments washed off the Nevadan orogenic 

 belt. The Coast Ranges might thus be an element which was built 

 up and added to the continental plate from Mesozoic time onward. 



A puzzling feature, incompatible with this concept, is the oc- 

 currence in parts of the Coast Ranges of a crystalline basement 

 consisting of granitic rocks, shown by radiometric determinations to 

 be of mid-Mesozoic age like those in the Nevadan orogenic belt, and 

 their host rocks of earlier schists, slates, and marbles. In one area, 

 the San Gabriel A'lountains north of Los Angeles, radiometric 

 determinations prove the existence of rocks as early as pre-Cam- 

 brian, but these probably do not emerge elsewhere. 



Disregarding the cover of Tertiary rocks, the fundamental frame- 

 work of the Coast Ranges is an alternation of these areas of crystal- 

 line Nevadan basement with a basement of much deformed but 

 little metamorphosed Mesozoic rocks, especially the Franciscan 

 group which may have been formed on the western, or oceanic side 

 of the Nevadan belt. 



Basement of Franciscan age underlies all the Coast Ranges north 

 of San Francisco Bay and extends thence southward along the west 

 side of the San Joaquin Valley (Fig. 10). Southwest of this northern 

 Franciscan area is a long strip of Nevadan basement, termed Salinia, 

 which extends southward from Point Reyes and the Farallon Is- 



