CHAPTER XXI. 



DISTRICT OF PLAYAS OR SMALL VALLEY TROUGHS. 



Ascent of the spurs of sierra calitro. — Approach to the plata. — Deposit of conglomerates — Granitic axis. — 

 Stratified rocks on the west side of the plata. — Extent, surface, and boundaries of the playa. — Sand ridges 

 of the margin, their origin. — Palaeozoic rocks of the basin.— Volcanic rocks, serpentine and augite.— Thick- 

 ness AND DIP OF the ENCRINAL LIMESTONE. ThE PLATA IN THE BAINT SEASON, ITS FHTSIOONOMT. ChIRICAHUI MOUNTAINS, 



STRUCTURE AND TREND OF. ItS RELATION TO THE PINALENO MOUNTAINS. STRUCTURE OF PUERTO DEL DADO. PoRPHTRY 



DTKES. — DoS CABEZ05 VoLCANIC ROCKS IN CANON. VaLLE DEL SAUZ, LIMESTONE ROCK OF. ExTENT OF TALLET. 



FeLONCILLO HILLS. TrACHTTE AND BASALT OVERFLOWS. EXTENSION NORTHWARD. SUCCESSIVE ERUPTIONS. PtRAMID 



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 gravely 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 4,707 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 N. 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 back 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 CALITKO HILLS. 



These hills have been already noticed as the 2d range in the Piiialeno mountains. They 

 are well seen coming up the Gila at the mouth of the San Pedro.* The Gila caiions 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 tliree distinct ranges running N. by \V. parallel to each other may he seen in proximity, the Sierra Calitro being the 

 most easterly of these. The Pin ilono r.iiige is hidden from view. 



19 U 



