RE VIE I VS 641 



mile south of Point San I'edro, or about sixteen miles south of the 

 most northern point of the peninsula. At its exposure along the 

 shore line it is overlain by the basal conglomerate of the Franciscan 

 series, but a short distance from shore it becomes the surface rock and 

 covers an elliptical area whose major axis is ten miles, extending in a 

 northwest and southeast direction, and whose minor axis is four miles. 

 Thai this formation is newer than the crystalline limestone is indicated 

 by the fact that " farther south in the Santa Cruz range, the same 

 granite is found in irruptive contact with pre-existing terranes among 

 which crystalline linjestone or marble is prominent," and also by the 

 fact that a mass of marble was found embedded in the granite within 

 the area considered. 



The petrographic character of the Montara granite leads to the 

 conclusion that it cooled as a batholite. The mantle, which con- 

 sisted in part of the crystalline limestone was almost entirely removed 

 before the Franciscan series were deposited, which in turn were 

 removed before the Tejon-like sandstones were laid down. 



The Franciscan series occupy the greater part of the peninsula, and 

 are separated by the Merced Valley into a northern and a southern area. 

 Petrographically the series are divided into (i) A basal formation; 

 (2) The San Francisco sandstone; (3) Foraminiferal limestone; (4) 

 Radiolarian cherts; (5) Volcanic rocks. Of these the most important 

 are the San Francisco sandstone and the Radiolarian cherts. 



The San Francisco sandstone is that of the early writers. It forms 

 the bulk of the sedimentary rock of the series. Its original color is a 

 greenish or bluish gray, but it easily weathers to a yellowish brown. 

 In the field it presents a massive aspect "due to the thickness of the 

 beds and the obscurity of the bedding planes." 



The Foraminiferal limestone is described as having "a fairly con- 

 stant petrographic character throughout the terrane, although it is 

 found at more than one horizon." It is generally traversed by two 

 sets of veins — calcitic and siliceous. The latter are often parallel to 

 the bedding of the limestone, and may be contemporaneous with it. 

 The limestone itself is thought to be a chemical precipitate. 



The Radiolarian cherts are of a dull reddish-brown color, are 

 several hundred feet thick, composed of sheets of chert from one to 

 four inches thick alternating with partings of shale, and occur locally 

 in lens-like masses. Petrographically they occur as true jaspers, as 

 rocks of a flinty or hornstone character, as rocks that are easily scratched 



