136 GEOLOGY OF THE SIERRA ESTRELLA. 



of hills, which, coming in from the northwest, form elevated ridges, round which the Gila turns 

 after having received the Salinas waters. These ranges are parallel, and having sloping plains 

 to the west. They are granitoid in structure, of a blackish color externally ; but the fresh 

 surface displaying white and pink felspar, intermingled with clear crystals of transparent quartz, 

 adularia occurs in veins and crystals. The veins of reddish felspar run northeast and north- 

 west, intersecting each other. Quartz veins run north 30° east at the most elevated summit of 

 the trail ; in the pass epidote occurs in seams, and a black syenitic granite, which easily rusts and 

 decomposes. The rocks at this point are mostly felspathic, with but little quartz or mica ; 

 occasionally masses of fine-grained granite occur without any granular structure; thick beds of 

 gray felspar rock with rhombic cleavage. Cubes and prisms of titanium ore occur in the quartz 

 veins which cut through the felspathic granite, constituting the mass of the mountain. As a 

 whole, it may be looked on as a series of granite hills running parallel at eight to twelve miles 

 apart. The ascent from the west for fifteen miles being very easy, when the summit is 

 gained, a caiion passing through the range and opening into a flat plain often miles wide; this 

 summit plain is elevated, the trail leading direct east through another caiion (in a second range 

 of hills) and then easily descending some sixteen miles to an alluvial plain at the base of the 

 mountain. The whole of this travel, forty miles, is over a loose, stony conglomerate and felsj^ar 

 sand, without water, and is known as the Jornada of the Pimas plains. The mesquite, larrea, 

 green acacia, and the fouquieria, with the cereus giganteus, (pitahaya,) are the chief growth. The 

 cereus constitutes the only tree found 100 feet above the plain, upon which it never comes down ; on 

 the east slope, several echino-cactus and mammillaria were seen. The mass of this range is a 

 felspar granite rock, in places passing into protogine ; the first range crossed contained albite in 

 the granite, with large crystals of quartz, seams of epidote, and, in the canon, a blackish 

 granite verging into a basalt in appearance, close-grained and black on the outside. Grneissose 

 rock, dipping 25° west, lay on the left side of the caiion ascending it ; no metamorphic rocks 

 were observed on the east side of the range, which has received the name of Sierra Estrella or 

 Star mountains. 



In passing through the second canon, the more eastern range was observed to be made up 

 of a reddish granite, blackened on the weathered surfaces, containing large crystals of felspar, 

 (orthose,) adularia in veins and crystals, and cut up by threads and seams of quartz, as obaerved 

 in the western range. A third range, lying eastward of the trail and between it and the river, 

 has a similar constitution, and it was in this latter that the specimens of titanium ore were 

 collected. The summits of these ranges cannot be less than 2,000 feet above the plain; they 

 can be approached quite closely, owing to the level slope of the plain at the base, and have not 

 the rolling foot hills which mountains usually possess. Taken as a whole, this Jornada is a 

 double inclined plain, sloping east and west, away from a central line — the line being this series 

 of hills running N. W., rising abruptly out of the sloping plain; this character is very 

 common to the topography of this region, the hills not possessing any valleys proper to them, 

 but appear to rise out suddenly from the strata (gravel and conglomerate unsolidified) which 

 repose uncomformably round the base, and contain fine quartz and felspathic detritus, with 

 occasional debris of basalt and porphyry ; fine granitic sand is found in the dry beds of the 

 arroyos ; several of these granitic ranges lie parallel — each single crested and presenting no 

 trace of sedimentary rock on their sides. The ascending slope (western) is 15 miles long, and 

 the eastern or descending slope 24 miles. 



A section of Sierra Estrella is given on plate IX, figure 3. 



