CHAPTER XXL 
DISTRICT OF PLAYAS OR SMALL VALLEY TROUGHS. 
Ascent of the spurs of sierra calitro.—Approach to the playa.—Deposit of conglomerates —Granitic axis.— 
Stratified rocks on the west side of the playa.—Extent, surface, and boundaries of the playa.—Sand ridges 
of the margin, their origin.—Paleozoic rocks of the basin.—Volcanic rocks, serpentine and augite.—Thick¬ 
ness AND DIP OF THE ENCRINAL LIMESTONE.-The PLAYA IN THE RAINY SEASON, ITS PHYSIOGNOMY.-CHIRICAHUI MOUNTAINS, 
STRUCTURE AND TREND OF.-Its RELATION TO THE PINALENO MOUNTAINS.-STRUCTURE OF PUERTO DEL DADO.-PORPHYRY 
DYKES.— Dos CABEZOS -VOLCANIC ROCKS IN CANON.-VaLLE DEL SAUZ, LIMESTONE ROCK OF.-EXTENT OF TALLEY.- 
PELONCILLO HILLS.-TRACHYTE AND BASALT OVERFLOWS.-EXTENSION NORTHWARD.-SUCCESSIVE ERUPTIONS.-PYRAMID 
HILLS, THEIR STRUCTURE.-VaLLE DE LOS PLAYAS, SOIL AND SANDSTONE OF. 
To reach the Playa de los Pimas from the San Pedro, at Tres Alamos, the trail passes over 
a low range, the subsidence of the Santa Catarina, joining by a spur the Calitro range of hills, 
and both merging into the general level of the extensive plateau; in so doing it divides into 
several spurs, and over the one which trends furthest south this road passes. The ascent, 14 
miles to the summit, is easy, and is over beds of granitic clay and gravel, deposited very 
nearly horizontal, from 80 to 100 feet in thickness—dipping west a few degrees north, at a very 
small inclination. The upper beds appear to be of the coarser material. This formation is 
not found on the descent of this spur, which is composed of the exposed surface of the stratified 
rock. This is the eastern limit of this unconformable tertiary stratum of desert gravel. 
The summit over which the trail runs has an elevation of 4,107 feet, and is a plane surface 
for several miles ; the axis is granitic, very abundant in yellowish felspar ; a quartzose compact 
conglomerate lines both sides of the axis, dipping 3ST. 30 W. and S.E.; not more than 40 feet of 
thickness of these beds are exposed. Further down the eastern slope of this spur limestone is 
exposed, with a bare, thin soil covering it, the dip eastward ; a few degrees south, and upon 
the hack of the strata for some miles the road lies. This bed is fossiliferous—encrinital—and 
in places cyrstalline and metamorphic ; a characteristic of it is the mode of weathering, leaving 
rough elevated surfaces—the whole strata being fissured throughout, and the cavities filled by 
white amorphous carbonate of lime. This rock is also very argillaceous, and weathers irregu¬ 
larly, leaving a brownish yellow, clayey surface, caused by the removal of the superficial crust 
of carbonate of lime. 
GEOLOGY OF THE CALITRO HILLS. 
These hills have been already noticed as the 2d range in the Pinaleno mountains. They 
are well seen coming up the Gila at the mouth of the San Pedro.* The Gila canons through 
them and exposes vast masses of red sandstone, which constitutes the great bulk of the hill. 
The whole range is capped with limestone ; the lower carboniferous beds, which, further to the 
south and east, is found in the basins and lower country, are here uplifted 2,500 feet above the 
level of the river bed, and broken through and tilted at opposing angles by basaltic and amyg- 
* Where three distinct ranges running N. by W. parallel to each other may be seen in proximity, the Sierra Calitro being the 
most easterly of these. The Pinaleno range is hidden from view. 
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