F. N. Guild — Petrography of the Tucson Mountains. 313 



Art. XXXY. — Petrography of the Tucson Mountains, Pima 

 Co., Arizona; by F. 1ST. Guild, University of Arizona. 

 (With Plate IX.) 



The Tucson Range of mountains is located directly west of 

 Tucson and is about twenty miles long with an average width of 

 about seven miles. It consists of a series of jagged peaks extend- 

 ing nearly north and south, the higher ones of which are esti- 

 mated to have an altitude of 4000 feet above sea level. The 

 approach to the main line of peaks is over a series of low-lying 

 rounded knolls devoid of all vegetation except a few cacti and 

 other stunted growths characteristic of an arid region. 



Petrographically quite a variety of rocks are represented 

 which are almost entirely eruptive. There occur, however, in 

 places, remnants of the original quartzites and limestones 

 through which this great mass of lava has broken. On the 

 west side of the range and southwest of Tucson is an elevated 

 plateau of an area of one hundred square miles or more, con- 

 sisting entirely of these limestones and quartzites. It is quite 

 level and the beds are exposed only along its edges and near 

 the center where the uplifted strata form two small buttes, 

 consisting almost entirely of crystalline limestone tilted to an 

 angle of about forty-five degrees. 



It is the purpose of this paper to describe from a petro- 

 graphical standpoint the eruptive rocks without discussing their 

 geological relations. The question of names and classification 

 is not taken up, the writer considering descriptions of more 

 importance. With this introduction, they will be described in 

 the order of their relative abundance. 



Phyolite. — The main line of jagged peaks referred to above 

 is made up of this rock, varying in color from a dark red to 

 nearly white. Phenocrysts are inconspicuous, not very abun- 

 dant and rarely exceed three millimeters in diameter. They 

 consist of quartz and less abundant orthoclase. Under the 

 microscope, the quartz is found to occur in rounded masses 

 corroded by the grounclmass with frequent inclusions of the 

 latter in the form of bag-shaped inlets. Black dust-like inclu- 

 sions and glass with gas bubbles are common. The feldspar, 

 although much decomposed and containing opaque inclusions, 

 still shows the characteristic cleavage of orthoclase. The 

 groundmass in the darker varieties is too much altered to show 

 any characteristic texture. In the southern portion of the 

 district, however, material is sometimes met with of sufficient 

 freshness to admit of satisfactory study, and here it is found 

 to have a cryptocrystalline structure. Specimens of it are 

 frequently found possessing faint flow lines, sometimes vis- 



