314: F. N. Guild— Petrography of the Tucson Mountains. 



ible to the unaided eye, but usually requiring a microscope 

 to be seen. (Fig. 1, Plate IX.) Occasionally dark shredded 

 masses occur which may have been originally mica. All vari- 

 eties contain irregular inclusions of varying size, sometimes 

 two inches across, of a red jasper-like substance or of sandstone 

 or quartzite. Frequently also rounded patches are met with 

 which under the microscope are found to be made up of quartz 

 and feldspar in equidimensional crystals, which may represent 

 areas of more complete crystallization of the groundmass. 



Associated with these more typical rhyolites are large masses 

 usually of a light yellow to buff color, lacking all phenocrysts. 

 They correspond to rocks which have been variously called 

 felsite, felsophyre, granophyre, etc. They sometimes break 

 with conch oidal fracture, but are more often too coarse-grained 

 to show this characteristic. Under the microscope, quartz, 

 feldspar and sometimes shreds of mica can be seen in the 

 coarser varieties. The liner-grained types are made up entirely 

 of cryptocrystalline material in which none of the constituents 

 can be determined. 



Rhyolitic Tuff. — Associated with the outflow forming the 

 main rhyolite peaks, there were probably formed masses of 

 volcanic ash. The greater portions of this have been washed 

 away, but occasionally where geological conditions have been 

 favorable some of this material has become consolidated into a 

 compact rock of light gray color of sufficient strength to be 

 used extensively in building. Underlying Sentinel Peak in 

 places there are small masses of it which have been held in 

 place by the basaltic outflow. Under the microscope it is 

 found to be made up of fragments of quartz, feldspar, glass, 

 etc. It is interesting to note that the quartz has the same 

 kind of inclusions as the quartz phenocrysts in the rhyolite 

 described above. 



Andesites. — Several types of this rock varying greatly in 

 appearance and texture occur. They may be grouped rather 

 roughly as follows : 



1. Light-colored andesites containing phenocrysts of mica 

 or hornblende or both and of feldspar. 2. Dark-colored 

 andesites of non-porphyritic texture. 3. Vitrophyric andesites. 



The first variety covers an area only slightly less in extent 

 than the rhyolites and constitutes the material of the low-lying 

 knolls previously referred to. It has usually a mottled appear- 

 ance not unlike that of some granites. The feldspar is pure 

 wmite and the groundmass varies from white to greenish gray. 

 The chief variation is in the black ferro-magnesian minerals, 

 which are most often biotite, but in some localities hornblende 

 predominates, while in still others the black phenocrysts are 

 quite inconspicuous. Under the microscope the feldspar is clear, 



