22 ARTESIAN WELLS ON THE LLANO ESTACADO. 
a pole ten feet long was thrust into them, the bottom was not reached." Five miles further 
west another spring was found, and still another large one four miles beyond, which gave rise 
to a stream. 
Lieutenant Garrard reports numerous localities of good water at the eastern base of the Guada- 
lupe range, south of Captain Pope's main line of survey. He found it in natural tanks, about 
thirty-five miles southwest from the head-springs of Delaware creek, and in several water-holes. 
These were all filled with rain-water, and, from its quantity and other —— it was con- 
sidered probable that a supply would be found at all seasons of the year.! 
rtesian wells.—The frequent occurrence of springs with such a volume of water as Captain 
Pope describes, shows the existence of vast quantities of subterranean water, which probably 
flows below or between the nearly horizontal strata of the Llano. In fact, it would appear that 
the Colorado and Brazos rivers, or their afluents, which take their rise at or in the borders of 
the Llano, are supplied by springs gushing out in the ravines. These facts are of great value 
and importance, and leave scarcely a doubt that water can be obtained on the Llano by boring. 
The nearly horizontal or slightly undulating position of the strata, and the variation in their 
mineral character which is known to exist, especially the alternation of compact beds of lime- 
stone, sandstone, and clays, present all the requisite conditions for retaining the water under 
pressure. All descriptions of the strata of the Llano agree in placing at their base a thick form- 
ation of red clay, and above it more sandy and porous rocks. We have, therefore, in the clay 
an impervious substratum, serving to retain the subterranean water in the more porous beds 
above. These pervious beds are believed to alternate with impervious ones, and thus the under- 
ground flow of water is confined. In order, however, that water should rise to the surface, or 
overflow from artesian borings, the strata which confine it must be inclined, or formed into 
basin-shaped depressions, so that one portion of the water-bearing stratum is higher than the 
point pierced by boring. On the Llano the strata, although described as horizontal, doubtless 
present not only a considerable degree of inclination towards the east and southeast, as shown 
by the gradual inclination of the surface, already described; but flexures or extended bends and 
depressions in the strata doubtless exist. Such variations indem a horizontal plane are found 
in every extended sedimentary deposite, even where the strata have not been disturbed by con- 
vulsive changes of the earth’s crust. Indeed, a perfectly horizontal mass of strata over broad 
areas is nimowa to geologists. The occurrence of the dry salt lakes and the ponds on the 
surface of the plain; the depressions which become filled with water during the rainy season 
only, all indicate the existence of corresponding depressions in the underlying strata. I do not 
find any evidence in Captain Pope's observations, or in those of his party, to justify me in the 
conclusion that the strata of the Llano are upheaved, dislocated, or distorted by the uprising of 
the Guadalupe mountains. This appears to be Captain Pope's impression, based, perhaps, upon 
the statement of Mr. Marcou, who did not visit the district Captain Pope explored. The con- 
figuration of the Llano, its topography and generally level character, are, to me, insurmountable 
objections to this view. The intrusion or the uplift and folding of such a mass of mountains as 
the Hueco range and the Guadalupe, in whatever manner it took place, would have produced 
most decided flexures and abrupt folds of the strata of the Llano, so as to completely destroy its 
character as a plain. The configuration of the region is opposed to the opinion that the moun- 
tain ranges are more recent than the strata of the Llano. A disturbance of this nature is not, 
however, necessary, as has already been shown, to the production of the uneven condition of 
the strata requisite for obtaining water by boring. The gentle inclination of the plain and the 
strata from the mountains eastward, amounting in the whole breadth of the Llano to at least 
400 feet, according to Captain Pope's measurements, is sufficient to give water at the surface of 
the eastern margin from a depth equally great, provided the permeable strata were fully charged, 
and the water did not suffer retardation by friction. It is possible, however, and indeed most 
probable, that the difference of elevation is much greater than 400 feet; and, if so, water could 
Report of Captain Pope—letters of Lieutenant Garrard, p. 63. 
