136 
GEOLOGY OF THE SIERRA ESTRELLA. 
of hills, which, coming in from the northwest, form elevated ridges, round which the Grila 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 hut 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 he 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 canon passing through the range and opening into a flat plain of ten miles wide; this 
summit plain is elevated, the trail leading direct east through another canon (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 felspar 
sand, without water, and is known as the Jornada of the Pimas plains. The mesquite, larrea, 
green acacia, and the fouquieria, with the cereus giganfceus, (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 alhite 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. Gneissose 
rock, dipping 25° west, lay on the left side of the canon 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 he 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 observed 
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 he less than 2,000 feet above the plain ; they 
can he 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, 
hut appear to rise out suddenly from the strata (gravel and conglomerate unsolidified) which 
repose uncomformahly 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. 
