(lABBROS. 101 



of actinolite iu nearly parallel arrangement, giving- the mass a schistose 

 character. The microscope shows a little white mica, chloi'ite, and serpen- 

 tine in the rock. Included in the actinolite are also grains of titanite, brown, 

 somewhat pleochroitic prisms of rutile, and small zircons. Long, thin, dark 

 inclusions, arranged parallel to the cleavage of the amphibole, are perhaps 

 also rutile. The following analysis shows the composition of this rock: 



Loss at 100-, H-0 aOO" 



T>0B8 above 10C-, H-O 0. 91fi 



Silica, SiO' 50. 4;i7 



Alnmiiia, Al-0^ 8. 18:} 



Chromic oxide, Cr-C o. 4S0 



Ferric oxide, Fe-C 1. 0.11) 



Ferrous oxide, FeO (i. 285 



Manganous oxide, MiiO 0.21:! 



Lime^ C'aO 11. .TiO 



Magnesia, MgO... 17.628 



Soda, Na-0 2.983 



Potiissa, K-O 0.503 



Total 100.304 



The atomic ratio deduciblo from this analysis is II- : R' : R" : Sirr 

 10!) : IJylf] : 0.539 : 3.3G2. 



G.\BBR0IT1C PSr,lM>ODIAB.\SE. 



While a tendency to the development of tlie clinopinacoidal cleavage 

 of the pyroxenes of the pseudodiabase has been already noted, character- 

 istic diallage is somewhat rare. ( )iie occurrence is known on Bagley Creek, 

 at Mt. Diablo, the great mass of which is composed of zoisite-pseudodiabase 

 and phthanites. It is a dark-green rock, composed of granules of from one 

 to two millimeters in diamet-^r. Fresh feldspar and the grayish-green, al- 

 most metallic luster of the diallage cleavage surfaces arc visible with the 

 naked eye. 



Under the microscope the diallage is monochroitic, nearly colorless, 

 and carries inclusions in the direction of the best cleavage. The slide con- 

 tains a little uralitic hornblende. In a few places a decomposition-product, 

 similar in ap[)earance to the ferruginous cement of the sandstones, appears 

 to have formed from the diallage. The feldspars are clear and give extinc- 

 tions as high as 30"^ on each side of tlie twinning plane, indicating the pres- 

 ence of labradorite. 



