GEOLOGY OF THE SAN JOSE DISTRICT 265 



Range. It is a very dark gray, from the very large amounts of 

 black ferro-magnesian minerals present. These are in prisms 

 and grains surrounding masses of nephelite. No feldspar is 

 apparent to the naked eye. 



Microscopic Characters. — The microscope reveals, as the 

 component minerals of this rock, apatite, titanite, magnetite, 

 biotite, hornblende, augite, orthoclase and nephelite, in the order 

 of their formation. The calculation of the mode by optical 

 methods makes plain that only the last four play an important 

 part in the rock. Hornblende is greatly in excess of all the 

 other constituents put together. Orthoclase is very subordinate. 

 Biotite is present in very small amounts. Its occurrence is sig- 

 nificant. It is not found anywhere in the district except in the 

 most basic forms of the nephelite syenite. The percentage 

 composition is as follows : 



Nephelite 13.2 



Orthoclase 5.6 



Hornblende 60.5 



Augite 13.9 



Biotite 2.2 



Magnetite 0.4 



Titanite 2.8 



Apatite 1.4 



The average grain is .16 mm. The texture is granitoid. 



The mineral nephelite is younger than the orthoclase, with 

 which it is in part intergrown. It appears in large areas, extin- 

 guishing at the same moment in which the feldspar and dark 

 silicates are enclosed. It is very clear, except for inclusions of 

 glass and fluid cavities. Its polarization tint is deeper than that 

 of the feldspar. It is shown to be uniaxial and negative. 



Orthoclose is largely interstitial between the crystals of am~ 

 phibole. No plagioclase was observed. 



The anhedrons of amphibole are gathered together to form: 

 large patches in the slide. Intergrowths with pyroxene are not; 

 found. The mineral is brown with pronounced pleochroism. 

 a= hght brown, t)= seal brown, C=deep brown. The optic angle 

 is large. The clinopinacoid is the plane of the optic axes. Ex- 

 tinction 21°. These properties are those which we should expect 

 from barkevikitic hornblende. 



