146 ELEVATED PLATEAU — STRUCTURE OF PLATA. 



daloidal tracliy tic intrusions ; a layer of amygdaloid being found in some places concealing the 

 limestone. Saddleback niountain in this range is an instance of this tilting of the strata at 

 short intervals ajiart, giving the hollow-backed appearance to the movintain from which it 

 derived its name. This range is remarkable for the flat summits of the hills — forming large 

 plateaus of several acres in extent. Its direction is north 45° west. The exposure of sandstone 

 beneath the limestone capping cannot be much less than 1^000 feet. This rock has already been 

 referred to the Devonian system, and is the oldest sedimentary rock met with on the survey. 



The range has an average breadth of 10 miles,- and cannot be less than 100 miles in length. 

 Its course along the right bank of the San Pedro is above 70 miles in a straight line, and it can 

 be traced by the eye more than 20 miles north of the Gila. 



GEOLOGY OF THE PLATA DE LOS PIMAS. 



This playa is a level plain of white sandy clay, glittering with white quartz and felspar par- 

 ticles, which have not undergone attrition sufficient to round them. To the eye it appears an 

 uniform flat, but it rises gently toward the north, where it is continued into the valley between 

 the Calitro hills and the Mount Graham or Piiialeno range ; it is from this northern prolon- 

 gation of the playa that the fine quartz particles are, in all likelihood, derived, inasmuch as vast 

 masses of loose quartz rock is found at the slope of the Calitro hills, and in the beds of the 

 arroyos leading from that range on its east side. 



Where the playa impinges on the base of the mountain ranges, several small, low sand masses 

 are formed, the effect of winds on the plain collecting the sands together. 



The low range of the Sierra Calitro, just described, over which the trail runs, forms the western 

 boundary of the playa. On the eastern border lie the Chiricahui mountains. The granitic axis 

 of both, being similar and in connexion, form the basin in which lie the stratified rocks of the 

 palaeozoic period. They lie conformable to the primary bed, and are, in a few places, cut 

 through and displaced by veins of serpentine and dykes of augitic rock, which are more evident 

 on the eastern than on the western margin of the j^laya. 



The slope from the Chiricahui to the playa is gentle and rolling, cut up in a few places by 

 deep, dry arroyo beds ; it is here that the sedimentary strata are displayed in part. The lime- 

 stone is exposed in the bed of a creek about two miles before the pass in the mountain is reached ; 

 it is here metamorphic, and filled with seams of white carbonate ; in other respects it is similar 

 to that forming the tabular slope of the west side of the playa ; the beds slope down into the 

 playa and form the superficial rock at least upon its margin. Higher up the creek bed there is 

 exposed a bed of metamorphic sandstone rock. Sixty feet in thickness of this was exposed ; it 

 dips under the limestone, though its contact with it was not observed ; ascending the base of 

 the mountain felspar, porphyry, with quartz veins, constitute the foot hills. 



The strata met with in the playa, and occupying the whole valley, are : 



1st. Coarse red conglomerate, cemented by a gypseous and calcareous paste, including rounded 

 pebbles, mostly of jasper, occasionally of augite, syenite, and other plutonic rock. On the 

 Chiricahui slope this conglomerate is 90 feet thick. On the western limit of the playa it exceeds 

 100 feet ; but as it is not fully exposed its total depth there cannot be declared. 



2d. Whitish sandstone grit, metamorphic, and in places possessing double line of cleavage. 

 This bed reposes on the conglomerate, and was not exposed on the western edge of the playa ; 

 on the eastern it was 60 feet in thickness. As this bed is much thicker further eastward, it is 

 probable it may be here also, but its limits are covered up by superficial detritus. 



