GEOLOGY OF THE SAX JOSE DISTRICT 263 



The soda augite which occurs so sparingly in this variety of 

 the nephehte syenite is clear green in color. Its crystals are 

 small and irregularly bounded. They are conspicuously pleo- 

 chroic, apple green to brown. Titanite is found as a rule close 

 beside the magnetite. Reaction rims between the latter mineral 

 and feldspar are occasionally seen. Apatite is rare. Epidote 

 is present in minute amounts. 



(c) Mesa Verde Type. 



This variety of the nephelite syenite occurs immediately to 

 the east of the Mesa Verde. Seen in the hand specimen it is 

 of a dark gray color from the large amounts of ferro-magne- 

 sian minerals contained in it. Clear, greasy nephelite may be 

 recognized, and slightly weathered feldspar, apparently not so 

 abundant, its cleavage surfaces having a vitreous luster. Clear 

 yellow grains and crystals of titanite are present. The dark 

 silicates occur as black masses which, except for an occasional 

 six-sided crystal of hornblende, cannot be identified. The rock 

 is fine-grained with a granitic texture. 



Microscopic Description. — Under the microscope it is found 

 that nephelite, orthoclase, hornblende and augite are the essential 

 minerals. Magnetite, apatite and titanite are the accessories. 

 By measurements with the micrometer eye-piece the miner- 

 alogical composition or mode of the rock was found to be as fol- 

 lows : 



Nephelite i8 



Orthoclase 12 



Hornblende 40 



Augi te 23 



Magnetite 2.7 



Titanite 2 



Apatite 1.2 



The average grain is .2 mm. The structure is granitic with 

 all the minerals more or less completely xenomorphic. From 

 the above table the great prevalence of the dark silicates over 

 the feldspar and nephelite together may at once be noted, as 

 well as the excess of nephelite over orthoclase. 



The orthoclase is in crystals inclining to a prismatic habit 

 which average .4 mm. in diameter. They may at times be 



Annals N. Y. Acad. Sci., XIV, March 22, 1904 — 18. 



