F. N. Guild — Petrography of the Tucson Mountains. 315 



usually striated, and as shown by the extinction angles on the 

 twinning plane appears to be an acid plagioclase. The biotite 

 is quite fresh and of the usual dark yellow-brown color. Horn- 

 blende has become darkened by alteration and is often quite 

 opaque. The groundmass is crystalline and made up mostly 

 of feldspar with some magnetite and shreds of the dark 

 silicates. 



The second variety is found in the northern portion of 

 the district near the edge of the mountains about twelve 

 miles from Tucson. It varies greatly in both megascopic 

 and microscopic structure in different parts of the same 

 mass. Its general appearance is more like that of a diabase 

 except in portions where phenocrysts of feldspar appear. It 

 is very dark with a slight green tinge weathering red. Por- 

 phyritic texture is not conspicuous and may be megascopically 

 absent. Under the microscope, however, the rock is found to 

 consist of crystals of plagioclase, pyroxene, and biotite in a 

 variable groundmass. In some portions the distinction between 

 groundmass and phenocrysts is very marked, the groundmass 

 being typically andesitic, while in other parts there is com- 

 paratively little difference in size between the constituents of 

 the groundmass and the phenocrysts. The pyroxene is light 

 yellow-green in color with high extinction angle and non-pleo- 

 chroic and rarely occurs in crystals longer than one millimeter. 

 The plagioclase phenocrysts are usually somewhat larger, 

 ordinarily clear but sometimes opaque from decomposition. 

 The biotite appears in rather small crystals compared with the 

 other phenocrysts and is of a light yellow-brown color with 

 darker borders. In altered specimens the dark-colored con- 

 stituents have decomposed into yellow non-pleochroic masses. 



The third variety, or vitrophyric andesite, is also found in 

 the northern portion of the district as a low rounded ridge not 

 more than one hundred feet above the surrounding country. 

 It is also a pyroxene mica andesite, and is distinctly porphyr- 

 itic, the phenocrysts occupying fully one half of the entire 

 mass of the rock. Black mica and feldspar are very conspicu- 

 ous and occasionally ortkoclase crystals eight millimeters in 

 length showing well-formed Carlsbad twins occur. The 

 groundmass varies from a nearly black to light gray transpar- 

 ent glass. Under the microscope the feldspar is found to be 

 of plagioclase and of an unstriated variety. It frequently 

 possesses zonal structure and is often much broken, appearing 

 in angular fragments. The biotite is in fresh hexagonal plates 

 and irregular shreds. Pyroxene is light green and shows high 

 extinction angle. Magnetite is present in the usual quantities. 

 The groundmass is isotropic and filled with what appear to be 

 small fragments of the phenocrysts and very small crystallites. 



