36 GEOLOGY OF LLANO ESTACADO. 
since they not only receive their supply of water at remote points from this source, and from 
the melting of the mountain-snows, but they are sheltered from the air and sun, and protected 
from the consequent evaporation. 
When the strata fulfilling these conditions, and out-cropping at high altitudes, can be traced 
continuously without interruption, the boring of artesian wells will always be successful. 
I will therefore, with this brief sketch, proceed to examiné*the Staked Plain, with a view to 
the supply of water which can be procured from the two classes of springs above referred to. 
Geological structure of the Llano Estacado.—The upper geological formations of this plain, 
according to Mr. Jules Marcou,* are, first, the Jurassic, 700 feet thick; and, second, the Trias, 
3,000 feet thick. 
Beginning at the upper surface of the plain, the Jurassic consists of super-soil, white lime- 
stone with fossils, yellow sandstone, blue clay, compact white limestone, oolitic white sand- 
stone, and red and variegated soft sandstones: the Trias, of red friable sandstones, with red 
and gray clay—in a word, variegated with interpositions of beds of sandstone and dolomite, or 
magnesian limestone. 
The strata of the Jurassic and Trias outcrop at the eastern base of the Guadalupe mount- 
ains, and at an altitude of 800 feet above the highest line of the Llano Estacado. From their 
line of dislocation they have a very gradual dip to the east-southeast, and are uninterrupted 
as far, at least, as the headwaters of the Colorado of Texas, at which point the white oolitic 
limestone of the Jurassic outcrops, and from beneath it issue the springs forming the sources 
of the Colorado. 
These strata, alternately permeable and impermeable to water, fulfil all the conditions neces- 
sary for the success of artesian wells. They are cut through by the Pecos, which interposes 
between the ‘‘Llano’’ and the Guadalupe mountains, down to the white oolitic limestone of 
the Jurassic, which forms the bed of that river. In boring, therefore, upon the Staked 
Plain, it would be necessary to pass entirely through the whole of the Jurassic above this 
limestone, or a distance of from four to seven hundred feet. 
In confirmation of the belief that boring would be thus successful, I quote from the report of 
Mr. Marcou.t ‘‘These rocks of the Llano Estacado (the Jurassic and Trias) dip gently to 
the east-southeast, and as the heads of the strata outcrop at the foot of the Rocky mountains, 
called here Sierra Guadalupe, Sierra Sacramento, &c., in boring artesian wells on any point 
of the Llano, abundant columns of water would be found to gush out over this immense plain ; 
so that the want of water is not an objection to the establishment of a railroad on the Llano 
Estacado, for it may be obtained anywhere.’’ 
The upper surface of the “Llano” is very gently undulating, and contains many shallow 
basins, which fill with water during the rainy season—the months of August and September. 
The basins are so shallow, and so large a surface of water is exposed to evaporation, that these 
ponds are dry düring a great portion of the year. 
It would be easy to dig wells or tanks in the beds of the lakes, and by conducting into them 
by ditches and drains all the water which falls upon the surface, a very large supply could be 
accumulated. It is impossible to say how much, but certainly stffficient to supply all travelling 
purposes for the whole year. This process would be somewhat cheaper than that of boring, 
but the supply of water would be less certain and much less abundant. 
Wells, also, dug at any point of the plain, would supply water which has percolated through 
the loose permeable soil, and which is retained by the impermeable stratum of limestone below. 
The average depth of vada wells would probably not exceed sixty feet, although at some points 
it might reach one hundred and fifty. 
In all respects the artesian wells would be preferable. The water would, from what I have 
stated, be delivered at the surface, and would probably rise far above it; and instead of arrange- 
— preliminary report by Mr. Marcou upon the collection. Preliminary Reports, 1st edition, 8vo. Washington, 1854. 
