26 THE OEOLOaiCAL STRUCTURE, ETC. 



are well displayed between Dennison, Fort Worth, and Waco. 

 The beds decompose on the surface into rich sandy soil of highly 

 absorbent properties. These Lower Cross Timber Beds are the 

 same as those Dakota Sands of Kansas, Nebraska, and Dakota, 

 and are equally valuable sources of artesian water supply, since 

 they possess unusual capacities for imbibition. The Lower 

 Cross Timber Series is the source of the water in the wells at 

 Pottsborough, Dallas, and Midlothian ; the depth at which water 

 was obtained in these wells was under 700ft., and the supply 

 was moderate, as might have been expected. Had these wells 

 been continued to the Paluxy and Trinity Sands no doubt better 

 yields would have been obtained. 



To the north of Colorado River the outcrop of the Trinity 

 Sands is in part concealed by the Tertiary beds, which form 

 what is known as the Llano Estacado or Staked Plain. 



The Llano Estacado merits a little detailed description on 

 account of the part it plays in supplying water to the Trinity 

 Sands, in addition to that which is absorbed by the portion 

 exposed directly to the surface. In this respect it bears a close 

 resemblance to the Desert Sandstone of some parts of Queens- 

 land. 



The Llano Estacado is a smooth plain of at least 50,000 

 square miles in extent, and is surrounded on almost every side 

 by a precipitous escarpment often visible for a distance of 50 

 miles. This escarpment is said to form one of the most 

 remarkable topographical features of the North American 

 continent. The western margin has an altitude of 5,000ft., 

 while on the east it is only 2,500. The geological structure of 

 the Staked Plains is very simple, consisting of beds of uncon- 

 solidated porous waterworn sands, gravel, and silt. These beds 

 dip practically at the same angle as the surface of the ground, 

 which is to the seaward, at about 20ft. per mile. The Llano 

 Estacado is, with the exception of occasional ponds formed in 

 the depressions after copious rains, singularly devoid of water. 

 Running Water, Dickens County, is said to be the only stream 

 on the Staked Plains. A writer describes it as " a bright spark- 

 ling stream that suddenly breaks out of the ground, ripples over 

 pebbly porous bottoms for a distance of 10 miles, and then 

 mysteriously disappears like many other streams west of the 

 Pecos River." 



