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



fresh for satisfactory study. To the naked eye the porphyr- 

 itic character of the rock is not at all apparent. Under the 

 microscope, however, it is found to be made up of distinct 

 porphyritic crystals of abundant feldspar, considerable pyrox- 

 ene, and much less olivine in a semicrystalline groundmass 

 consisting of a felt of magnetite and dark matter which reacts 

 feebly under polarized light. 



Quartz basalt. — This unusual type of basalt is found in the 

 extreme southern end of the mountain range as a portion of 

 the promontory-shaped hill two miles southwest of the San 

 Xavier Mission. The greater portion of the outflow consists 

 of the compact basalt already described with cellular and 

 scoriaceous modifications. On the extreme eastern slope an 

 abundance of the quartz-bearing variety appears. Quartz 

 is the only mineral that can be detected with the naked eye. 

 Aside from this porphyritic constituent the general character 

 of the rock from both a megascopic and microscopic stand- 

 point is the same as the compact varieties described above. 

 The quartz occurs as rounded and semi-angular grains, rarely 

 more than six millimeters in length. Under the microscope 

 the quartz appears clear, much fractured and quite free from 

 inclusions of all sorts. That the quartz is primary and not 

 due to secondary filling of cavities is inferred from the fact 

 that the grains each consist of but one individual as shown by 

 the extinction. This is not the case where previously existing 

 cavities have been filled by infiltration. Amygdaloidal fillings 

 have been observed in this same rock and they present a struc- 

 ture quite different from the quartz in question. Basalts con- 

 taining quartz have been described by Diller,* Iddingsf and 

 PirssonJ from various localities in the United States, and by 

 Andreae§ and Lacroix|| from other regions. By most of these 

 writers their origin has been discussed and they have been 

 held to be of primary origin. Some, like Lacroix, have, how- 

 ever held them to be inclusions, quartz grains caught up from 

 lower rocks and held in the magma. 



DESCRIPTION OF FIGURES, PLATE IX. 

 Figure 1. — Rhyolite, showing flow-lines in the groundmass, ordinary light, 



18 diameters. 

 Figure 2. — Vitrophyric andesite, near Gila Bend. 

 Figure 3.— Basalt, Sentinel Peak, 45 diameters. 

 Figure 4. — Basalt, near San Xavier, 45 diameters. 



Figure 5. — Porphyritic basalt, showing crystals of augite, 18 diameters. 

 Figure 6. — Agate, "under polarized light, showing complicated structure, 



found as amygdaloidal filling in porphyritic basalt (c), 45 



diameters. 



* This Journal, vol. xxxiii, p. 45, 1887. Bull. U. S. Geol. Surv. 79, 1891. 



f This Journal, vol. xxxvi, p. 208, 1888. Bull. U. S. Geol. Surv. 66, 1890. 



X Bull. U. S. Geol. Surv. 139, p. 129, 1896. 



% Zeit. dent. Geol. Gesell, 1892, p.' 824. 



I Enclaves des rockes volcaniques, Ann. Acad. Macon, vol. x, 1893, p. 17. 



