NATURAL TANKS OR RESERVOIRS OF WATER. 15 
The plain between the Hueco and the Guadalupe mountains is broken by several short ridges 
or isolated mountains of moderate height, and shown on the map under the names of Sierra de 
los Alamos, Sierra Alto, and Los Cornudos. As these mountains are probably all granitic, we 
may conclude that the foundation of granitic rock, or its subterranean surface, is not far from 
the surface of the plain; or, in other words, that the valleys in the granite are comparatively 
shallow, and that there is not a very great thickness of stratified or paleozoic formations rest- 
ing in them. The topography and the probable Carboniferous strata of the Guadalupe moun- 
tains lead me to consider this highly probable, and to regard the country between the two 
ranges—the Hueco and Guadalupe ranges—as a broad axis or summit of granite and metamor- 
phic rocks overlaid by sedimentary accumulations, similar to those of the Llano Estacado. 
Natural tanks or reservoirs of water.— The natural tanks or wells which were visited are 
an interesting and peculiar feature, and seem to be placed in those desert plains for the comfort 
of the traveller. "These tanks are found principally in the Hueco mountains, but occur at Los 
Cornudos and other similar localities. "The tank in the Hueco mountains, near the trail, is 
described by Mr. Bartlett as a great cavity in the rock, containing about fifty barrels of pure, 
sweet water. This cavity was covered by a huge boulder weighing some hundred tons, and its 
lower surface was only four or five feet above the water.! 
It is difficult to understand by what action the rocks, if of granite, were symmetrically hol- 
lowed out so as to form these well-like reservoirs. It would seem, from some of the descriptions, 
that they were in sedimentary rocks or strata of sandstone; and, if so, decomposition may have 
been the principal cause of their formation. Large cavities and irregularly-formed depressions 
are very common on the surface of granite—and probably this is the character of the tanks—the 
water being overshadowed and preserved from rapid evaporation by piles of rocky débris. Mr. 
Byrne states in his Journal that the water is not found in the tanks alone, but that there are 
numerous holes and crevices in the mountains, which contain enough for the use of travellers. 
This would indicate that the water collects in the irregular depressions of the rock, and not in 
holes worn out by the action of running water. 
Similar reservoirs of water were found in the mountains of the African deserts by Bayard 
Taylor, who describes one in the Beyooda desert, as a vast natural hollow in the porphyry rock, 
in the centre of a basin or valley near the top of the mountain. ‘‘ The water is held in a tank; 
it is from twenty to thirty feet deep, and as clear as crystal. The taste is deliciously pure and 
fresh.’’? 
It is very probable that some of these reservoirs are constantly fed by the condensation of 
moisture upon the surrounding rocky summits; and others may be springs supplied through 
fissures in the rock, the evaporation from the surface being so rapid as to prevent their over- 
flow. A great quantity of water is derived from the melting of snow which falls on these 
mountains and on the surrounding plains in the winter season. 
1 Bartlett, Personal Narrative, i, p. 134. 
Journey to Central Africa, &c., by Bayard Taylor, p. 414, 10th edit.: New York, 1°54. 
