GEOLOGY OF THE SAN JOSE DISTRICT 259 



bands with great regularity, so as to cause the rock to resemble 

 gneiss. 



The San Jose nephelite syenite carries a comprehensive series 

 of the commoner rock-making minerals at nearly every locality 

 which was visited. Feldspar, nephelite, amphibole and pyrox- 

 ene are the usual essential components of the rock. Mica is 

 rare as an accessory constituent, while titanite, magnetite and 

 apatite are common. No sodalite, cancrinite, garnet, calcite, 

 olivine, lavenite or wollastonite was found. The essential min- 

 eralogy lies between widely varied amounts of orthoclase and 

 nephelite with soda augite and brown barkevikitic hornblende. 

 Plagioclase is rarely present. Mica is never an important 

 constituent. 



Varieties occur where by the disappearance of nephelite the 

 rock is a typical syenite. Again by the coming in of plagio- 

 clase it approaches litchfieldite. One variant from the normal 

 type very low in orthoclase consisting principally of nephelite, 

 hornblende and augite marks the passage over to the ijolites. 



The San Jose nephelite syenite bears some resemblance to 

 the Red Hill, New Hampshire, rock, but it does not carry soda- 

 lite. It differs from the Litchfield and Dungannon occurrences 

 in its field habit, for it is never so coarse as these rocks are. 

 Nor is it except in rare cases so poor in the dark silicates. It 

 is unlike the Dungannon rock by reason of its large content of 

 orthoclase, and as being so poor in mica. It closely parallels 

 some of the more acid varieties from Magnet Cove, but its dark 

 silicates, though sharply automorphic, are not developed in such 

 perfection. Again the basic portions of the Mexican rock 

 might almost pass for certain facies of the Beemerville nephe- 

 lite syenite. The latter is commonly darker by reason of the 

 dusty iron inclusions which crowd its feldspars. This character 

 is never seen in the San Jose specimens. The rock was no- 

 where observed to grade into porphyritic varieties as would 

 seem to be the case at Beemerville. 



The nephelite syenite in the San Jose district has not suffered 

 metamorphism. It never displays a schistose structure. The 

 effects of mechanical strains are not shown in thin sections. 



