GEOLOGY OF THE SAN JOSE DISTRICT 271 



The crystals of orthoclase are shorter and stouter. They 

 occur as carlsbad twins and are much altered and kaolinized. 



The largest crystals of orthoclase do not attain a diameter 

 greater than i mm. They are xenomorphic rude prisms which 

 show marked pleochroism from light green to brownish-yellow. 



The quartz is in xenomorphic rectangular and square pieces 

 which give the characteristic uniaxial cross with one or two 

 rings. They may be distinguished from the orthoclase squares 

 by their glassy unweathered appearance. The quartz is seen 

 to make up but a very small part of the whole rock. 



Biotite, in small patches not above .2 mm. in length, is still 

 rarer. Magnetite is found abundantly and small lozenge-shaped 

 individuals of titanite are met in the slide. Some zircon is also 

 present. 



An analysis of the* San Narciso type of the andesite is given 

 below, as follows : 



SiOg 62.31 



AUO3 f 18.63 



Fe^O^ 2.38 



Feb I S3 



MgO 60 



CaO 5.91 



Na,0 4.97 



K2O 3.52 



PA 07 



HgO'io 16 



Hpig" ..., 07 



99.98 



The calculation of the standard mineral composition or norm 

 gives the following percentages : 



Quartz , 8.9 Diopside 4. 1 



Orthoclase 20.6 Wollastonite 2.5 



Albite 41.9 Magnetite 3,5 



Anorthite 18. 1 10. 1 



Sal 89.24 



The rock is persalane and belongs to order V canadare. It 

 is domalkalic and dosodic, laurvikose. 



