1XXX1V REPORT OF THE STATE GEOLOGIST. 



The third and possibly best explored collecting area is that of the 

 Trinity Sands. 



This bed, the Trinity or Upper Cross Timber sands, is the base of the 

 Lower Cretaceous system, and is the great water-bearing bed east and 

 south of the Central basin. In its many exposures and from the mate- 

 rial brought up from it in boring, its composition is shown to be clear 

 white grains of quartz, slightly rounded to much worn, containing a 

 few grains of red and black chert. It is for the most part practically 

 free of soluble mineral matter, and the water derived from it is often of 

 excellent quality. From its position, character, and extent, it forms 

 a most important member in the geology of Texas. The water which 

 falls upon the exposed edge of this belt is carried under the limestone 

 of the Grand Prairie plateau, and part of it breaks forth in a system of 

 great springs which extend from Williamson County by Austin, San 

 Marcos, and New Braunfels, toward the Pecos. These springs are natu- 

 ral artesian wells, which owe their existence to the fault lines caused by 

 the disturbances, already alluded to, which formed the Balcones. The 

 remainder of the water continues its course below the overlying forma- 

 tions, and can be reached at almost any point east and south of the 

 Grand Prairie to the border of tne Basal Clays of the Tertiary. Wells 

 are very numerous and vary in depth with distance from catchment 

 area from one hundred to two thousand feet. They can not be named 

 in detail here, but the principal boring has been at Fort Worth, Dallas, 

 Waco, Austin, Taylor, San Antonio, and in Somervell, Coryell, Hood, 

 and Bosque counties. These prove that artesian conditions exist, and 

 there can be no doubt that wells bored in suitable localities will prove 

 successful. 



West of the Grand Prairie plateau we find the Central Basin region, 

 which is principally occupied by strata of the Paleozoic formations. 

 The eastern and southern border of this area is plainly marked by the 

 scarp of the Grand Prairie. Its western border is not determined fur- 

 ther than that in Texas it is terminated by the Guadaloupe Mountains 

 in El Paso County. In its topography it shows a gradual elevation 

 toward the west, most usually, however, in a series of steps which rise 

 one above the other, having the ascent facing toward the southeast and 

 a long gentle slope toward the west, the average rise being less than 

 eight feet pei mile. 



At the edge of the Staked Plain, which is a newer formation super- 

 imposed upon these, there is an abrupt elevation of from two hundred 



