ASPECT OF THE PLATA — VEGETABLE GROWTH. 147 



3d. Encrinital limestone, metamorphic and crystalline in its lower beds ; roughish ; brown 

 on the exterior ; deeply fissured, the cracks filled with amorphous carbonate of lime ; blue in 

 tlie interior, and in some beds highly fossilif'erous. Perhai)s this is the superficial rock of the 

 plaja ; none other crops out upon the surface or border. It is hard to fracture and weathers 

 slowly, and is largely exposed on the western slope of the playa, where it forms for miles the 

 bare rock of the surface, so lightly covered with soil that only cactus and fouquieria can support 

 an existence. 



On the west boundary this rock dips southeast 7° ; on the east side it dips southwest 12°. 

 Tlie thickness on the western side is 150 feet; on the east it is covered up so that not more than 

 40 feet is exposed. But it is probable the total thickness is not less than 200 feet. 



During tlie excessive rains in the rainy season, the whole or the greater part of the playa 

 bottom is covered by water to a depth from a few inches to some feet. During some large portion 

 of the year there is a small lake in the centre, and such has been noticed by some travellers, but 

 tlie continued evaporation ultimately dries up all surface liquid, and leaves no traces but the 

 soft clay bottom, marked in places with the rippled lines of the recent lake; the soft texture 

 of the soil allows the foot to sink several inches down ; and, on the margin of the playa, a 

 collection of fine sand or beach of angular quartz ; without vegetation on its surface it resembles 

 an extensive field freshly ploughed and rolled, over whose heated surface the mirage depicts 

 its beautiful and tantalizing lakes of great extent.* 



The jjhysiognomy of this district is peculiar, and different from what is presented in similar 

 situations in the Atlantic States. There the interval between two mountains is a distinct 

 valley or trough, and the approach to the mountain is broken by a gradual swell and rolling 

 country; but here there is no valley — it is an uniform flat, running abruptly to the base of the 

 hill, which thus stands boldly out, and once reached is immediately ascended. It would appear 

 as if where these submarine elevations occurred that the shore actions must have been of some 

 power and duration to deposit such an amount of sedimentary strata and detritus, that the 

 whole valley proper has been filled up to an even level line ; and but little alteration has been 

 produced in this, save where the mountain streams have cut deep arroyos in its sides. 



It is this horizontality of the detrital beds which has produced the playa by reason of the 

 inefficient drainage of the waters; there being no decided fall in any direction, the water lodges 

 in the subsoil, forming springs. 



The vegetation on the Calitro hills is that of a desert ; a variety of black walnut is found in 

 the canons near the summit^ but the descent is quite sterile, being mostly an exposed bare rock, 

 (limestone.) Fouquieria and agave, with palmetto, but no traces of mesquite, are found on the 

 descent to the playa. 



The slope to the playa on each side is very gradual, so that although a space of twenty miles 

 intervenes between the Sierra Calitro and Chiricahui, yet the playa proper has not a greater 

 breadth than eight or ten miles. It is difficult to estimate its length, as its boundaries north 

 and south are not defined — stretching northwards between Mount Graham and the Calitro hills 

 and rounding Chiricahui, both north and south, and spreading into the elevated jjlain country 

 to the south and east. 



•^ Lieut. G. Andrews, 3d Art., commander of the escort accompanying the survey, informed me that on l>is return with his 

 company to Fort Yuma, crossing this plain early in December, he found a small lake, a mile wide and perhaps three miles 

 long, four to six inches deep— the accumulation of the summer rains on the slopes and over the surface of the valley. 

 This shows what a large volume of water is available if means were taken to preserve it. 



