20 SAND HILLS.—SALINE LAKES. 
wagons next to impossible. On approaching from the east or west, these hills bear every 
appearance of the sand ridges along the coast of South Carolina and Florida, and the first hard 
wind will destroy the traces of the most numerous party."! Mr. Byrne states an interesting 
fact, that water is found in abundance among these hills, resting in basins of the purest sand. 
It appears that water could be at any time procured by going about half a mile from the road 
among the hills. This fact is somewhat difficult to explain, but it is probable that the water 
occupies a depression in the subjacent rock, and that it serves to retain the sand as it blows over 
the plain; and the sand, in turn, protects the water from the complete evaporation which would 
probably result if the surface was fully exposed to the air. The fact that the sand hills have 
accumulated in that place, shows the existence of some natural obstacle, or a peculiar configu- 
ration of the surface; for when driven before the wind, sand does not come to rest, except under 
the lee of some obstacle. In all cases where sand progresses before the wind in drifts, one of the 
slopes—that towards the wind—is more gentle than the other; that slope turned from the 
wind, or on the lee side of the drift, being the most abrupt. This sand is doubtless derived from 
a superficial covering of drift upon the Llano, or from the sandstone strata. Mention is fre- 
quently made of a hard gravelly surface before the belt of sand hills was reached, and I do not 
doubt that this sand is swept out from the gravel and small boulders by the wind, and perhaps 
derived in part from the disintegration of the soft sandstones of the upper part of the mesa. 
This view is supported by the examination of the specimen of the sand which Captain Pope 
brought, in No. 77. It is very clean and white, and consists chiefly of silicious grains, much 
rounded by attrition. They are colorless, and some are transparent. When treated with 
chlorohydric acid, effervescence takes place, and continues for some moments, showing the pres- 
ence of a considerable amount of carbonate of lime. This indicates that the sand is derived from 
the decomposition of the white calcareous sandstone of the region. The sand of the hills on the 
Colorado desert also contains a very considerable amount of carbonate of lime. It is proba- 
bly derived from the calcareous cement which invests the pebbles, forming a conglomerate 
on the surface of the plain, above the alluvial formations of the Desert. This conglomerate is 
Tertiary, or Quarternary ; ; and the sand is swept from it by the wind, leaving a clean surface of 
pebbles, like a pavement. 
Saline lakes. —A very peculiar and interesting Dai of the surface of the Llano and the 
table-lands west of the Guadalupe range, is the presence, in the lowest parts of these plains, of 
shallow ponds or lakes, which are generally sulphurous or saline. Of this character are the 
Mustang springs, consisting of several lakes or pools with highly saline water. The Ojo del 
Cuerbo, or Crow spring, at the western base of the Guadalupe Pass, is in connexion with two 
small lakes covering four or five acres of ground, which are sulphurous; and about one and a 
half mile west of them two salt lakes are found.? Mr. Byrne also describes a dry salt lake, 
two and three-quarters miles across, near camp 22 on the Pecos river. It was covered with a 
salty efflorescence, and, on digging through it and the soil to a depth of two and a half feet, 
water was obtained highly charged with uil. 
It thus appears that lakes or ponds of this character are quite numerous on the hini or in 
the vicinity of the route. Captain Pope's general observation doubtless gives the true expla- 
nation of the origin of most of them. He states that they were dry in the month of March, but 
that they probably contain water during the rainy season. The salt is derived from the sub- 
strata or porous rocks of the plain, being dissolved out by the rain water in its percolation 
through them, or passage over the surface; thus, each successive evaporation of the water in 
the low grounds adds to the accumulation of salt. 
I find in the collection a specimen (No. 72) taken from the bed of one of the salt lakes, 15 miles 
west of the Guadalupe mountains. It is a light, porous earth, nearly white, and resembling 
some forms of travertin. It crumbles readily in the fingers, and dissolves serials) in chlorohy- 
dric acid with violent effervescence. The solution thus obtained contains a large quantity of car- 
! Report of Captain Pope, p. 9. ? Report of Captain Pope— Diary of J. H. Byrne, p. 55. 
