Anderson.] 



Point Reyes Peninsula. 



1 29 



Too little is yet known for any satisfactory comparison, but it is 

 worth while to record a few observations upon this topic which 

 may be in the line of reaching some conclusion. Granites have 

 been more or less perfectly described from several points in 

 western California south of the area of Point Reyes peninsula, as, 

 for example, from Montara Mountain, 1 Monterey and Carmelo 

 Bays, 2 and from near San Luis Obispo, 3 with the exception of some of 

 the "Montara granite," which Professor Lawson calls a hornblende- 

 biotite granite. All the types yet described are rather acid rocks in 

 which biotite is the dominant fero-magnesian mineral. Quartz is 

 generally abundant and the feldspars are both orthoclase and plagio- 

 clase. It is a remarkable fact that the porphyritic type with large 

 phenocrysts of orthoclase, twinned on the carlsbad law, characterize 

 three of the four regions thus far studied. This type has been 

 well described from the vicinity of Monterey by Professor Lawson. 

 In many respects this occurrence is paralleled by the porphyritic 

 facies, if it may be so called, that occurs at the southwest angle 

 of the Point Reyes area. Dr. Fairbanks mentions the same char- 

 acter as belonging to some of the granite near San Luis Obispo. 

 The most common accessory mineral, found plentifully in at least 

 three of these localities, is titanite. Other characters more strictly 

 chemical will doubtless be recognized in time which shall prove 

 more satisfactorily the essential identity of these granites, but at 

 present these features are noticeable and have more or less weight. 

 Northward along the coast or from its near vicinity no granites of 

 a similar nature have been described, though there is probably an 

 equivalent and contemporary rock to be found in the Klamath 

 Mountains. A coarse-grained, quartzose, biotite granite makes up 

 a large part of the central core of the Trinity Mountains at the 

 head of the south fork of Salmon River. This granite has been 

 intruded into a series of Paleozoic sedimentary, and older schistose 

 rocks mentioned in another paragraph. The porphyritic character 

 has not been noticed there, but further northward in the Siskiyou 

 Mountains a similar granite occurs at Ashland peak and eastward, 



1 15th An. Rept. U. S. Geol. Sur., 1894, pp. 408-415. 



2 Bull. Dept. Geol. Univ. Cal., 1893, pp. 9-18. 

 3 Jour. Geol., Sept. -Oct., 1898, p. 567. 



