ASPECT OF THE PLAYA-VEGETABLE GROWTH. 
147 
3d. Encrinital limestone, metamorphic and crystalline in its lower beds ; rougbish; brown 
on the exterior ; deeply fissured, the cracks filled with amorphous carbonate of lime ; blue in 
the interior, and in some beds highly fossiliferous. Perhaps this is the superficial rock of the 
playa ; none other crops out upon the surface or border. It is bard 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°. 
The 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 the 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 
the 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 physiognomy 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 G-raham and the Calitro hills 
and rounding Chiricahui, both north and south, and spreading into the elevated plain country 
to the south and east. 
° Lieut. G. Andrews, 3d Art., commander of the escort accompanying the survey, informed me that on his 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. 
