UNIVERSITY OF CALIFORNIA PUBLICATIONS 



BULLETIN OF THE DEPARTMENT OF 



GEOLOGY 



Vol. 4, No. 18, pp. 425-430 ANDREW C. LAWSON, Editor 



AN ALTERATION OF COAST RANGE 

 SERPENTINE. 



BY 



A. Knopf. 



In the vicinity of North Berkeley bold massive croppings of 

 a curiously weathering" rusty red rock form conspicuous features 

 of the foothill slopes. They frequently attain a height of 40 or 

 50 feet, and owing to the striking character of their superficial 

 alteration, readily attract attention. Under the action of the 

 weather they yield cavernous, honeycombed masses of great var- 

 iety of form, gnarled and twisted, and often of fantastic fash- 

 ioning. 



The unweathered facies of this rock consists of an aggregate 

 of chalcedony and rhombohedral carbonates, whose relative pro- 

 portions may vary within rather wide limits. Highly siliceous 

 phases are common, while the other extreme is represented by 

 phases indistinguishable from crystalline limestones. In the 

 typical rock, however, the carbonates are dominant, and the 

 chalcedony subordinate. The structure is often peculiar and 

 characteristic, and is due to the intimate lenticular interlamina- 

 tion of the two main constituents. On a fresh fracture the rock 

 is colored a sort of terra-cotta red, and is seamed with small 

 vitreous quartz veinlets, and bunches and veins of white waxy 

 chalcedony. Chromite occurs in numerous small specks, and 

 upon isolation gives a good chromium reaction with borax. Occa- 

 sionally a deep green mineral of lamellar habit can be found. 

 Cleavage flakes show the emergence of a negative acute bisectrix 

 of small axial angle. The extinction is parallel to a well-defined 

 cleavage, the relief low, and the birefringence feeble. These 



