428 



University of California Publications. 



[Geology 



tions can be traced wherein the carbonate has completely sup- 

 planted the serpentine. It is not unusual to see the characteristic 

 antigorite structure displayed by carbonate. 



Certain larger veinlets of carbonate also occur, and usually 

 show a well-developed rhombohedral cleavage. The usual strong 

 differences of relief are displayed, and from the very high index 

 for 0, dolomite and magnesite are suggested. 



The remaining portions of the sections are composed of con- 

 fused aggregates of serpentine, which render inferences as to 

 their derivative origin unsafe. Only occasionally is the charac- 

 teristic antigorite structure visible. No olivine, or structure cer- 

 tainly referable to its former presence, could be found. A few 

 mosaic polarizing veinlets of quartz seam the slides. 



Under medium high power some tangled trichite-like aggre- 

 gates can be discerned embedded in the carbonate and quartz. 

 They possess a metallic lustre, inclining to brassy. A straight 

 and relatively stout prism showed a hexagonal cross-section. 

 These characteristics serve to identify the mineral as millerite. 



Summarizing the petrographic details, we find that the rock 

 is composed of allotriomorphic plates of bronzite, often serpen- 

 tinized, and magnesian carbonates epigene upon both the serpen- 

 tine and the pyroxene. It, therefore, conforms to a partially 

 altered pyroxenite. 



From the rock at North Berkeley whose origin is thus appar- 

 ent, the series of changes involving the transformation to the 

 carbonate rock may readily be followed, and may even be seen 

 in the hand-specimen whose typical pyroxenite character is 

 macroscopically obvious. Throughout the Coast Ranges, how- 

 ever, the carbonate rock often occurs in isolated croppings, highly 

 weathered, or associated with serpentine in obscure relations, 

 hence the uncertainty which has existed as to its origin. 



The approximate chemical composition of the original unal- 

 tered bronzitite as computed from the mineralogical content is : 



Si0 2 = 60 



FeAj = 6 _ 8 

 AL0 3 J 



MgO = 28-30 



CaO= 2 



Sum = 100% 



