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



Fine flow-lines and perlitic cracks occur in places. This 

 variety of andesite is very common in southern Arizona and 

 frequently possesses flow-lines of remarkable beauty. (Fig. 2.) 

 In the upper part of the andesite the groundmass has become 

 entirely opaque through devitrification. 



Basalt. — Outflows of this rock occur at various intervals 

 along the edges of the mountain range especially west and 

 south of Tucson. They vary greatly in character and may be 

 grouped into the following varieties : 



1. Fine-grained olivine basalt. 



2. Porphyritic basalt. 



a. Containing phenocrysts of feldspar and augite, in a 



coarse-grained or doleritic groundmass. 



b. Containing porphyritic crystals of feldspar only in a 



basaltic groundmass. 



c. Containing feldspar, augite and olivine in an andesitic 



groundmass. 



3. Quartz basalt. 



One of the most prominent of these basaltic outflows is one 

 mile west of Tucson in the form of a symmetrical cone-shaped 

 mass called Sentinel Peak. Immediately northwest of this is 

 another irregular dome-shaped mass of the same rock. It is 

 further represented in two promontory-shaped outflows south- 

 west of the San Xavier Mission and ten miles south of Tucson. 

 These elevations are made up chiefly of the fine-grained type 

 of basalt in which none of the constituents can be recognized 

 with the naked eye. It is usually compact and free from 

 cavities, but occasionally is found quite cellular and even 

 scoriaceous in structure. The cavities are sometimes rounded 

 in outline with a diameter of one half inch or more, but are 

 more often drawn out by movements of the mass when in a 

 molten condition into irregular channels. These cavities are 

 usually empty, but are sometimes filled with gypsum or ara- 

 gonite. The predominating color is black, but deep red 

 varieties are met with, especially in the San Xavier outflow. 

 In places pressure and movement of the mass have developed 

 a schistose structure, the laminations frequently being nearly 

 vertical. This is especially noticeable on the dome-shaped 

 mountain mentioned above near the Carnegie Desert Botanical 

 Laboratory, and leads to the conjecture that the vent through 

 which the basalt escaped is located under it. 



Microscopically the rock is made up of numerous feldspar 

 rods crowded together and frequently arranged in flow-lines, 

 large amounts of magnetite and rather small quantities of 

 olivine. Glass is present in greatly varying quantities. The 

 olivine occurs mostly in rounded grains with a dark red halo 



