ft 



P A 1 F BANKS.] Geology of Point Sal. 75 



posed Knoxville age. About the same distance to the north, in 

 the vicinity of San Luis Obispo, are other large bodies of altered 

 peridotite, also intrusive in the Knoxville. As far as is known no 

 eruptives of this kind have been intruded in the Chico. The Point 

 Sal occurrence differs from all others known in the Coast Ranges 

 in regard to the large amount of gabbro accompanying it. Large 

 areas of olivine and norite-gabbros have been found by the author 

 in the Peninsular range in the southern part of the state, but there 

 is no means of correlating them with the similar rocks at Point Sal. 

 It seems more than probable that the rocks under discussion are 

 differentiated portions of the same basic magma which invaded the 

 Coast Ranges through their whole length at some time between 

 the deposition of the upper and lower Cretaceous. 



Comparison with Other Peridotites of the State. — Peridotites are 

 among the most widely distributed of the eruptive rocks of Califor- 

 nia. They occur through the Coast Ranges from a point a few 

 miles southeast of Point Sal northwest to Oregon. They are met 

 with along the western slope of the Sierra Nevada from Mariposa 

 County north until covered by the lavas. It is a well-known fact 

 that until recently geologists who have worked in California have 

 considered the serpentine of sedimentary origin. M. E. Wadsworth,* 

 who investigated some of the material collected by the old 

 State Geological Survey, is an exception, for all the specimens 

 which came under his observation he believed to be of eruptive 

 origin. In 1891, H. W. Turner f showed that the serpentines of 

 Mt. Diablo are altered eruptives, and expressed the opinion that 

 there is a genetic relation between the peridotite and the gabbro 

 found there. In a paper published in 1892 the author J advanced 

 the opinion, based on an examination of parts of the Coast Ranges, 

 that the serpentines are all altered eruptives. Later Dr. Palache|| 

 described the serpentine, and associated rocks of the Potrero, San 

 Francisco. There he found a lherzolite generally sheared in an 

 extreme degree, and accompanied by lenses of fine-grained rock 



* Lithological Studies. 



fBull. Geol. Soc of Am., Vol. II, p. 389. 



% Am. Geol., Vol. IX, p. 153. Eleventh Report of State Min. of Calif. 

 || Bull. Dept. Geol. Univ. Cal., Vol. I, pp. 161-179. 



