GEOLOGY OF THE SAN JOSE DISTRlCf 267 



I. Nephelite syenite, Baril type, Tamaulipas, Mexico. H. S. Washington, anal. 



II. Nephelite syenite. Red Hill, N. H. W. F. Hillebrand, anal. W. S. Bay- 

 ley, Bull. Geol. Soc. Amer. , III, 250, 1892. 



III. Eleolite syenite, Litchfield, j\Ie. L. G. Eakins, anal., Bull. Geol. Soc. 

 Amer., Ill, 241, 1892. 



IV. Eleolite syenite, Brogger. Syenitpegmatitgange, p. 33. 



When the norm of this rock I is calculated in the standard 

 salic and femic minerals the percentage composition is as fol- 

 lows •} 



Orthoclase 31-69 Magnetite 2.55 



Albite 37-73 Diopside 6.62 



Nephelite 11.64 Olivine 55 



Anorthite 7.78 Fern 9.72 



Sal gE.84 H.p 84 



The rock is therefore persalane, it belongs to order 6, rus- 

 sare, it is domalkalic viezzenase and dosodic, viezzenose. 



2. Dioidte 



The two principal areas in which diorite is exposed lie along 

 the road leading to the Vegonia Mine, at a distance of about a 

 mile from San Jose. The rock is also found in the gulch a 

 mile due northwest of the town. It is usually deeply weathered 

 and shows a rusty brown color in the field. The andesite is 

 younger than the diorite and surrounds it completely, sending 

 many small dikes into it. As before stated, the diorite probably 

 lies below the andesite over much of the country where erosion 

 has not revealed it. 



When seen in the hand specimen this rock has the appear- 

 ance of typical diorite. It has the granitoid texture. It is me- 

 dium-grained, and dark gray in color. Plates of biotite are 

 easily discernible through the rock. They are jet black, or 

 brownish-black, and large enough to allow cleavage pieces to 

 be scaled away. W^ith the biotite small amounts of another 

 dark silicate having no good cleavage may be identified. The 

 microscope shows this to be a pyroxene. Taken together the 

 darker constituents which are so prominent are not quite so 



1 " Quantitative Classification of Igneous Rocks," Cross, Iddings, Pirsson, Wash- 

 ington. The University of Chicago Press, Chicago, 1903. 



